I have managed to save everyone and get my team back with three rounds still on the clock, so it's not that bad. But. It does happen where the Gnomes just stand about and 'forget' to take their turn. That's a reload situation, really.
As for strategy. Before going through the hatch and triggering the turn-based countdown, prepare.
Speed and actions is the obvious choice, so Haste preferably as a twinned spell, bloodlust elixer is a prize here. Action Surge should be ready to go.
Also, have some lightning or cold damage spells or weapons ready in favor of fire as all the enemies will all be wet. A well placed lightning bolt will tear through the Sahuagin like there's no tomorrow. Mirror Image, Spirit Guardians are super handy to have. Also, bring your familiar if you have one. Just for occupying the enemy for a turn. Items that prevent getting restrained, typically boots, help a ton. Also, Misty Step and/or Dimension Door.
A potion of Speed works for haste, but you'll loose a turn to Lethargic, so keep that in mind. Also, never break the concentration of the caster who hasted.
Alright, through the hatch we go.
Step one, murder that guy by the door. The Sahuagin are total pushovers, but they have nets that will lock your people in place. Summon a mage hand on either Gale or Lae'zel or something. That mage hand has two roles: Pull the levers on the nearby doors, and, if that's done, tank a reaction by triggering an AOO. That will likely not happen, but you'll be glad to have it ready if you do need it.
You'll notice there's three rooms with captives, apart from the one you start in. Two have water on the floor, one has not. Enemies spawn from the water. That means in the 'dry room', no reinforcements will spawn. The character that has the most trouble with numbers or actions should go there. I send one person in each direction, keeping one near the ladder. I tend to pick Shadowheart for that, because I never invest in her speed; she's there to put down Guardians of Faith, Spiritual Weapons and CC anything that runs after the prisoners. I tend to cast Healing Word on hurt Gnomes and Dukes, too, but it's not really necessary.
Then. Take your time, and make sure everyone's rounds are as efficient as can be. You're in turn-based mode, time doesn't fly for you, treat it as such. Hope/plan to trigger Polearm Master/Sentinel if you can.
Some enemies will patrol in real time, wait for them to reach your character before ending a turn, and not using your action, for instance. If your character didn't rescue someone or kill a Sahuagin, you're doing it wrong. Keep an eye on the Initiative order. Always take out those that go before the captives first.
Tank reactions. It's better to have them AOO you than the 12HP Gnomes, so just take one for the team and move to where you need to be.
Then. Don't pick locks. I was a moron the first time around, picking the locks on the doors, casting Knock and breaking them with Disintegrate. That costs an action. Use the levers instead, they're free to use. There is one captive on a dentist's chair or somesuch, that one *does* take an action.
Be the last man out. More enemies will spawn in the water and will come for the captives. It's better if they come for you. If the Dry Room guy is done, rush him back towards the stairs and help there, snipe the monsters or something.
lastly, if all else fails - your guys can be resurrected. IF only one man makes it back, you can still save everyone. For a matter of coin, of course.
