In order to complete the game and locate the hidden treasure, you must visit all seven islands. However, before you can visit any island, you must first obtain an island gate key from a pirate ship. There are seven pirate ships which sail in various locations on the ocean. Only one of these ships will be stationary. You can only obtain a gate key from stationary pirate ships. If you attempt to challenge a pirate ship which is moving, you will have no choice but to complete the ship, and you will not be granted anything upon completion. At the start of the game, the pirate ship which contains the key to the Cuck Island gate will be stationary. After that, the next ship will not anchor itself until the boss of Cuck Island is defeated, and so on.
Defeating the bosses on each island is not enough to gain access to the treasure. You will need at least four other items: the three tablets, and Baido's tear drop. Two of the tablets and the drop can be found on various islands, while the last tablet is only accessible by finding and challenging the ghost ship, which sails throughout the entire ocean and can only be found after all seven pirate ships have been conquered.
Ship sequences[edit]
You can board a ship by manuevering your ship to the same location as the one you wish to board. If the ship is a green merchant ship, you can board it to learn pieces of information that may help you throughout the game. If the ship is black, it is a pirate ship, but you will only be able to obtain a key from the captain at the stern of the ship if it is anchored in one place. If it is moving, you will still have to fight all the way through the ship in order to return to your own, but you will not receive anything for your effort.
When challenging a pirate ship, all ships are composed of five screens. You can move freely back and forth between the first four screens, but the fifth and final screen will remain locked until you defeat the requisite number of Higemarus (pirates) have been defeated. This number appears in the upper left corner of the screen below your life points and counts down for every pirate that is defeated. When this number reaches 0, part of the bannister which prevents you from accessing the fifth screen will be removed, allowing you to pass. The action on the pirate ships is taken directly from this game's arcade predaccessor, Pirate Ship Higemaru. You must press 
to pick up a barrel, and then face a Higemaru and press again to release the barrel, and roll it into the Higemaru, thereby knocking it out. Some screens on the ship have a fixed number of Higemarus, while others continuously regenerate.
In this fifth screen, you will face the captain of the ship. All captains have the same abilities and pattern of attack, although the deck on which the fight takes place may be arranged differently on different ships. The captain will pace back and forth along the top portion of the screen, stopping at particular spots to jump up and down, or throw a pair of knives. In order to defeat the captain, you must continue to throw barrels at him until he disappears. The knives that the captain throws can cancel out your barrel throwing which leads to one of two strategies.
The safer, but more difficult strategy, is to manuever Momotaru so that he avoids the knives and still manages to throw the barrels at the captain between throws. This can be difficult to time, and may result in a lot of time spent moving around to avoid knives, and less time actually throwing barrels. The easier strategy is to line yourself up directly with the captain and intentionally take one knife throw before immediately throwing a barrel at the captain. You will absolutely take damage, but you are almost assured a clean shot at the captain. By intentionally taking hits, you can dramatically shorten the length of the battle, although it is a good idea to ensure that you have a good amount of life points stored up before going into the fight. If the pirate ship you entered was anchored and stationary, the captain will leave behind a key for you to collect before exiting the ship.
Island sequences[edit]
Once you have obtained an island key, you can then sail around looking for the precise island gate that the key corresponds to. If you do not have the correct key, you will only receive a message that you must have a key to enter, without being told which island you attempted to access. You will only learn the names of each island upon successfully entering them with the appropriate key.
Once you arrive, you will be presented with the entrance screen of the island. Each island is composed of a series of interconnected screens, which you can travel between as you cross the island. Each island has a boss that must be defeated in order to advance in the game. Island also contain one or possibly more items of importance, some of which are only accessible after the boss has been defeated.
While exploring the island, many screens will be occupied by enemies native to that island. Additionally, you will often find large quantities of rocks or barrels, which serve the same purpose as barrels on pirate ships; these are your primary method of attack. To clear a screen of enemies, you must pick up a rock or barrel, and face an enemy (or enemies) before throwing it at them. You will gain life points for every enemy that you defeat, and those points will double for every consecutive enemy that you defeat with the same rock or barrel. You may also find iron orbs which, when thrown, will smash through every rock or barrel in its way, along with any enemies.
On a couple of islands, you will find large monoliths. When you stand close to the monolith, unrecognizable letters will appear. Somewhere in the game lies a chart for you to collect which will provide you with a translation of the foreign alphabet to your own. These monoliths provide an important clue to the successful completion of the game, so remember where they are in order to reexamine them when you obtain the chart.