Items
Never buy
Do not buy items that improve relations with the Jesuits or Indians. These are the Golden Cross, Sacred Relic, Shrunken Head, and Carved Shaman Staff. They are completely useless because the Jesuits and Indians are always on good terms with you. This was not the way the designers originally planned the game: originally your relations were supposed to vary, but they had to release the game before they had a chance to implement this, and for some reason they left the items in.
Getting a perfect score on Swashbuckler
The number one thing to do is find Montalban. You only have so many years before you get old (especially before you find the longevity items yet) and Montalban is an extremely tough opponent. Once your health is "poor" (which happens when you hit 34 if you have both longevity items and your special skill is not Medicine), you can probably forget about beating him. (Do remember that you'll regain some health if you purchase a longevity item, so hope is not lost if you hit poor health but don't have the items yet.) Once you've beaten Montalban at his hideout, it's all downhill from there.
Note that Montalban's hideout spawns as soon as you beat him once, so by all means head straight there if you think you have a good guess as to where it is.
Since Montalban and Raymondo (who is a frequent source of your leads on Montalban) sail in Spanish ports, it helps to be on friendly terms with the Spanish so you can enter their ports freely, since you will usually have to be near Raymondo or Montalban to get hints as to their exact location from bartenders. Note that playing for the Spanish doesn't have to mean fighting against the other countries; this is elaborated upon below.
The other thing to do is don't get arrested or thrown overboard. (This is not so vital if you just reload from the autosave when this happens, but if you don't like "save scumming", this is of paramount importance.) The former wastes valuable time and the latter causes you to lose all the gold you're carrying on top of that. To avoid this, take care not to start a fight with insufficient crew. If you don't know how many men are on the ship you're up against, assume a lot. Also, if you're attacking a nation's ships near one of their cities, assume a pirate-hunter will spawn — sometimes you get in trouble not with the ship you attacked, but with the pirate-hunter that spawns after it. If you do wind up in such a position, it may be better to let the pirate hunter fire away at you as you try to escape, since even letting a ship sink is preferable to being jailed or lost at sea. If you have only one ship, though, you'll be lost at sea if it sinks, so you might end up having to fight the hunter anyway in a last-ditch effort to stay in the game.
Pirate hunters can be troublesome, so how do you avoid them? The easiest way is not to get in fights with national ships at all. It sounds crazy and runs directly against the common wisdom of plundering the plentiful Spanish ships, but it's perfectly viable to simply never attack a national ship (Raymondo/Montalban excepted) in the early game and it won't damage your chances of a perfect score at all. Enemy pirate ships are fair game since they don't spawn pirate hunters. One trick you can do if you're not coming across enough pirates is to go to a Pirate Town and encourage them to attack a city. They will send out a ship that you yourself can attack. If you wait a bit before you do this, one to three Pirate Privateers should show up as well. (This is because the game thinks it's an escort mission, and escorting a ship always causes privateers to spawn. These Pirate Privateers won't do anything, though.) The Pirate Privateers might not be carrying much gold, but you might find their ships useful, and the reputation you gain for beating them all is quite useful. In fact, pirates make great targets exactly because you can advance your standing with all four nations at once. You can find lots of Pirate Towns (and usually a few named pirates) along the coast of Central America and Mexico if you're looking for action and promotions. As your rank advances, the nations will forgive you for taking the occasional potshots at their treasure and payroll ships, though obviously this will reduce your chances for promotions/land grants.
At the start of the game, you may want to divide the plunder as soon as you have the opportunity to do so, so long as your crew will "eagerly" take their share (each pirate gets 50 gold or more). This will make recruiting much easier and, if you're very lucky, you can go without dividing ever again thanks to gold from named pirates, buried treasure, lost cities, and Montalban. When to try to divide, if they merely "gladly", "willingly", etc. take their share, cancel and go earn a bit more money or simply lose some crew in a battle (either a sea battle or land battle will do). Don't do something like take out Henry Morgan, because you'll get way more money than you need before dividing. It'll be more useful after dividing and help your next crew last.
Once you've divided the plunder once, you'll want to divide as infrequently as possible, possibly never at all, because it removes you from action for six game months, causing your character to age. However, don't fanatically avoid dividing the plunder either. You will need at least 200 (preferably 250 or more) to face Montalban at his hideout, and it makes sense to divide if you will be fighting Montalban soon but don't think you'll be able to maintain a large enough crew. Mutinies are also headaches, especially when they start stealing ships and taking some of your hard-earned gold with them. You can sell of all ships but one to avoid mutinies, but then you will be very vulnerable against powerful opponents like Raymondo and especially Montalban. It's best to always keep at least one spare ship and instead lose some crew through battle to avoid mutinies (since a mutiny is always caused by crew size — or hunger, but this is rarely a problem if you're careful), or divide if your crew is already too small to allow this.
Swordfighting
Choice of swords
If you're experienced at the swordfighting minigame, your choice of sword doesn't matter much. For example, simply choosing the Longsword every time will do fine in most cases, regardless of your opponent or his choice of weapon. However, you will almost certainly want a Cutlass against Montalban to counter his blazing speed. (This is assuming you are playing the 1.0.2 patch, where Montalban always wields a Rapier. Otherwise, Montalban will be wielding a Cutlass and you might possibly want a Rapier.)
Once your pirate's health falls down to "poor", you will likely need to use the Rapier to be able to attack effectively. Your defense will more than likely still be OK except against Montalban, though a strong opponent with a Rapier of his own will be quite difficult.
General strategy
If you just pound the attack keys, you will lose very fast. The way to fight is to watch your opponent's move, then use the correct response, and attack if you have an opening after the response. You can sometimes take the initiative with a thrust against a stationary opponent, but only do this if they are slow due to low Advantage.
Moves
The High Chop and Low Slash seem identical at first glance — both will advance you two squares if they connect, one if parried, and none if dodged — but in fact they are a bit different. The low slash starts out slower, but the recovery is quicker. Hence, especially if your character is slow, it may be best to thrust at a low slash instead of hoping to jump and slash/chop back. If your opponent has the low slash skill, more than likely you can forget about doing anything about his low slashes at all. You should prefer using the high chop to the low slash, since you'll be using it when your opponent is overextended and so its longer recovery is not a problem, whereas the low slash's longer setup time might be. (You might prefer a low slash or a thrust against an opponent with the Fast Jump skill, though.)
If your opponent is very fast, thrust instead of chop. Better to advance one space than to screw up and be driven back two. If you're fighting Montalban, you can virtually forget about chopping unless you've slowed him down a lot.
Taunting is usually useless because you're typically better off using the opportunity to score a hit instead of tweaking the advantage bar. Only do it when your opponent has become so fast that you can't get any attacks in, and then only after they overextend with a low slash or (preferably) a high chop.