Hi, my part is playing Crimson Throne, and we have an opportunity for an underwater sidequest.

We are currently 5th level. And our patron, has said the city will pay for whatever necessary equipment/spellcasting we need.

What do we need?

Piercing weapons aaaannnnddd....

We are me a Hereafter magus
Cleric of Crotues
Cleric of Shelyn
Paladin of Imodae
Superstitious Urban Barb
Crossbowman/Sniper
Sorcerer.

Underwater Crossbows are a must. Air Crystals, rings of swimming, Swim fins.

Scrolls (or Potions) of Water Brething
Potions of Touch of the Sea (due the short duration only used when you need quick action)

If they're really willing to pay for anything you need... you want necklaces of adaptation for everyone.
A wand of freedom of movement could also be extremely useful, as is anything that lets you get the aquatic subtype (and, ideally, a swim speed).

Hi, my part is playing Crimson Throne, and we have an opportunity for an underwater sidequest.

We are currently 5th level. And our patron, has said the city will pay for whatever necessary equipment/spellcasting we need.

What do we need?

Piercing weapons aaaannnnddd....

We are me a Hereafter magus
Cleric of Crotues
Cleric of Shelyn
Paladin of Imodae
Superstitious Urban Barb
Crossbowman/Sniper
Sorcerer.

Most of the essentials have been covered, though the party has within its means the ability to handle this endeavor without requiring much in the way of financial assistance form Korvosa.

As for mundane items, anyone desiring ranged attacks will of course NEED underwater crossbows. And swim fins are certainly not going to hurt anyone to have. Obviously spears, tridents, other piercing weapons of choice.

For anyone that can use them (either normally or via UMD), scrolls of alter self (and/or touch of the sea) are a cheap handy way to get a 30 ft. swim speed for 3 (or 1) minute(s) a pop. The Magus may simply want to scribe and memorize alter self.

Scrolls/potions of water breathing are one of the easiest ways to deal with breathable air (due to the duration), although with 2 Clerics, they could both simply pray to fill their 1st level slots with Air Bubble to cover the whole party on a minutes/level basis.

Otherwise, make sure your prepared casters don't prepare anything that doesn't work as well or at all underwater (invisibility spells and the fog/mist spells for example).

Happy Hunting!

If you anticipate that the DM will be playing up environmental factors (and really, if they aren't then why go underwater at all?), the following are worth considering.

Communication - Air Crystals stop working when you attempt to speak and water breathing does not explicitly grant any ability to communicate intelligibly underwater. You might resolve this simply by the use of bells, hand signals or slate boards or magically through the use of telepathic bond or tongues. Or just have everyone spend a skill rank on Linguistics and pick up Aquan, which can presumably be spoken underwater.

Darkvision - This is a must if you'll be operating at depth in open water. Any source of light will attract far, far too much attention and really, the dampening effect of water on light sources would make them less useful in the first place. On a related note, if anyone in the party makes significant use of stealth, be mindful of Watersense.

Charm Animals Wand - At your level many underwater threats will be classified as animals. A wand like this could prove unusually handy. If this is an undersea adventure, consider getting some blood in the water with the intention of charming some sharks as bodyguards.

Pressure and Cold Resistance - Deep water will freeze you and crush you. Magical protection from the former is a no brainer if you're going that deep. If that's not a given, anything that boosts Fortitude saves will buy you time at depth. Sadly, the only means known to me by which one can completely ignore pressure damage are in 3.5 books and I won't presume you're using those. But in a pinch, DR could probably be argued to provide protection.

Water Walk Scroll - A nice exit strategy for anyone who becomes incapacitated. Cast Water Walk on them and watch them drift back to the surface and presumed safety.

Rope - As silly as it sounds, there are a lot of environmental hazards underwater that fall into that category of 'Really, you intend to kill us with that?' Quicksand-like sediment, undertows, riptides, whirlpools, etc. Something as simple as rope can be of immeasurable use even underwater.

IMO scrolls of Water Breathing and Freedom of Movement are best. Water Breathing is better in scroll form than potion form because with one cast you can affect the whole party, but not if it's in potion form. Scrolls are also the cheapest form of Freedom of Movement available and are best for SHORT underwater adventures. If you're going to be underwater for days, however, a wand may be better than several scrolls. If a character is best with bows or crossbows, they should either find some way to make them work normally underwater (since Freedom of Movement doesn't affect ranged attacks underwater), or switch their fighting tactics to melee-oriented.

Well if we are going down the route of useful spell casting items for a relatively low level game which is, of course, underwater. Some things that I would personally make sure to get would be:
Wand of Summon Monster II (50 charges) 1500gp
*reason Squid
Potion of Alter Self 100gp
*reason swim speed, AC bonus, and darkvision
Scroll of Polymorph
*Just in case

If your GM allows really expensive items
Gills of the Shark 30,000gp
*continuous water breathing as per the spell

The problem here is breathing...