Underwater City Overview - Baldur's Gate 2
The Underwater City map (AKA City-of-Caverns) is an area that you can only access briefly during Chapter 4. You will be brought to this map by Saemon Havarian if you choose to accept his help after escaping from the Spellhold Dungeon and defeating Irenicus. If you refuse Saemon's help at this time then you can never access this map and all of the loot and EXP will be lost.
In my opinion the most important thing on this map is the Cloak of Mirroring item which is one of the best in slot cloaks. When wearing this cloak you're effectively immune to almost all spells cast at you including Beholder rays. I go out of my way and make sure I complete this map each and every playthrough just for this cloak.
Throne Room: When you first arrive at the Underwater City you'll be brought to this location to speak with the King. The King will ask you to kill the Rebel Prince and return his heart
Vendor: Here you will find a vendor that offers almost all temple services as well as having tons of different spell scrolls and items for sale.
Temple: If you're going to help the reasonable sahuagin out and have a talk with the Rebel Prince then you will have to visit this temple after you meet with the King. Here you'll be given the Rebel Orb which will allow you to peacefully communicate with the rebel faction.
Ambush: At this location you will be ambushed by a group of rebel sahuagin. They'll drop a Cloak of Mirroring and some magical ammunition but not much else of value.
Imp Puzzle: There will be 2 Imps on this platform that will ask you to play a game with them. It's a puzzle much like the one in Spellhold where you'll have to put the correct items into their appropriate chests. Successfully completing the puzzle will net you 18,500 EXP for every character in your party as well as Boots of Etherealness and Cloak of Protection +2. I have a picture below showing you the correct order for placing the items into chests.
Beholder: Here you'll meet a Spectator Beholder who is arguably the best NPC in the entire game. Essentially his story is he's stuck guarding the chest and he'll attack you if you mess with the chest - otherwise he'll talk with you. If you have Haer'Dalis or Jan in your party or if you main character has 13+ Wisdom (drink a Potion of Insight) you'll be able to work out a loophole in the Beholder's Contract which will earn you 15,000 EXP and the chest's contents. You'll find Sekolah's Tooth inside of the chest which is used to open the locked door leading into the Rebel Compound.
Undead: In this circular area you'll find numerous undead, the hardest of which will be a Sea Zombie Lord. Other than EXP there is no reward for defeating them all.
Rebel Compound: To complete the story quest involving the sahuagin city you'll have to visit this location and either agree to help the Rebel Prince or kill him for his heart; this is where you'll find the Rebel Prince. You will also find the hole that leads you to the Underdark in this area which you'll need the Magical Rope to use.