This is it; you're now looking at the treacherous waters that surround Lemuria. If you came here before, knowing Yepp's songs wouldn't help you because Poseidon at the exit of the Sea of Time would be there to smack you down with impunity behind his impenetrable forcefield.
But you have the Trident. Anyway, go up across two whirlpools. Go around and around the volcano until the current to your right stops, then sail across, around another whirlpool, until you get to a calm area. Go up across three whirlpools, and circle around the volcano until the current stops.
If you remember Yepp's first children's song, it ended with the "stars". Well, here's a rock shaped like a star. You're obviously going the right way. Go left across three whirlpools to a volcano, and use it to get past the current below you. Go across one whirlpool, into the calm area, then go left to the next volcano. Use it to get rid of the current below you temporarily.
You're now by the moon! You're nearing the end now. Go straight down the next few whirlpools to the volcano. Go around and around this one, then you can get past the current to the left. Go across to a calm area, then you can navigate the long chain of whirlpools going up. Use the volcano to stop the last current, then quickly sail north to the next area. Before you go any further, save. You might want to attempt the coming battle without any Djinn on standby to begin with - Poseidon's attacks are quite powerful and without the Djinn set to bolster your stats, you can die quite quickly. It doesn't mean you can't unleash them in-battle and summon, because you'll need to if you want to end this battle before you die. When you're ready, go to the right and up. Poseidon will appear.
Boss Battle: Poseidon
This is as far as you get without the Trident. If you notice, every single attack thrown against Poseidon before it's used does absolutely no damage thanks to his forcefield. This is why you equipped the Trident on your fastest character - the first move of the battle, you can break Poseidon's forcefield with it (it's a useable item under Items for the character that it's equipped to).
You'll want to pummel Poseidon with Psynergies and attack Djinn after this. Summon when you can, but try to keep Spark on Jenna set in case of downing, and the same with 4 Venus Djinn on Felix so he can use Revive whenever necessary. Any Waters of Life should be split between Sheba and Piers. Incidentally, if the Trident is equipped to Sheba, you may want to keep using it, since it can do 300-odd damage a turn, and Sheba is not really that useful otherwise unless she's unleashing Djinn and summoning.
Revive and Spark are necessary because Poseidon has one attack that will annoy you far more than any of his others. Watery Grave hits your entire party and has a chance of instant death. Ocean Fist will also be annoying, doing high damage to one person and also carrying a chance of knocking them down to 1HP. Since Poseidon has two attacks a turn, Typhoon Blow, a hit all Water-element attack, can come next, killing off whoever Ocean Fist hit. Poseidon also has Counter-rush, which makes him respond with a moderately powerful physical attack on anybody that hits him during this turn. This can essentially give him 5 attacks in one turn, so be careful.
If the battle goes anywhere near Poseidon's favour, Felix's PP will get eaten away at a tremendous rate, as he's trying to keep everyone alive. It's not worth repeatedly casting Odyssey, even though it can do a reliable 250-300 damage. Whoever has a Psy Crystal should use one on him when he runs out. Jenna's Healing Aura will also be a necessity, so make sure Jenna keeps at least 4 Mars Djinn set wherever possible. If Felix and Jenna die, you're in trouble.
On the plus side, Ulysses can do 1000+ damage, Thor can do about 800 and Meteor about 1000. If Felix gets a spare turn, use Ragnarok or Clay Spire. After a long pummeling, Poseidon will fall. Congratulations. The rocks after the battle will block off the exits, so go north and you'll emerge back on the world map. Save, then...finally...you can enter Lemuria.