What would be the best character to make for sub-sea and island-hopping adventuring? This would be a newly-rolled level 5 Pathfinder character.
I wouldn't say there's "one best," but there are certainly good ones. Mainly it's the same as being above water, except for mobility and breathing :-) I assume the reason you're starting at level 5 is your DM wants you all to have water breathing. You need to watch out for dispels, though.
An aquatic druid is a strong choice as @C.Ross mentions.
Consider a race with good swimming and/or a swim speed. Open Design's "Sunken Empires" has one, the half-merfolk. Or an aquatic variant; in Golarion there are the gillmen which is a human variant that can breathe water but can't be out of it for more than a day.
Warrior types that don't use heavy armor are good. High Str helps with that Swim skill. Also keep in mind that piercing weapons are the best underwater.
A cleric with the Water domain, e.g. of Gozreh, gets water breathing as a third level domain spell. There's one of those in my pirate-themed Pathfinder game.
As a summoner you can make an eidolon with a Swim speed, but have to use water breathing to let it breathe.
Don't try an archer or using thrown weapons, it basically doesn't work. Rely on casters for your distance punch.
Freedom of movement is your friend. A cleric with Liberation domain always has it as a 4th level domain spell AND gets it as a domain power. The Luck domain gets it as a domain spell.
Druids are extremely useful in an aquatic environment.
- An aquatic mammal (usually a dolphin) can make a great mount, even at first level.
- Once you can wildshape (which I believe would occur at 5th level), changing into an aquatic creature is extremely useful.
- I recommend the feat Wildspell so you can cast while in this form.
- Your environmental control spells are even more impressive on the open ocean.
- Control Water
- Control Weather
- Quench (burning ships are bad)
- You can summon aquatic creatures and water elementals which are very powerful on the open ocean.
Note: This answer is based heavily off the 3.5 rules. If anything here does not apply to Pathfinder, please edit or comment.
Explore the class features list of this thread, many classes have an aquatic variant. Most of the references there come from the book Stormwrack, which has an excellent discussion of aquatic campaigns.
Look at the aquatic race variants here.
From this flexibility, you get the option of running with functionally any class you want to play. If you are looking for a specific class, make sure to tailor your selections to the other player's "tiers" as an average variation of more than one tier can make for an uncomfortable party fit.
Do you have any other requirements beyond "aquatic" campaign? (This answer will be edited to fit, if more specific requirements are posted.)
I don't know pathfinder, but in DnD3.5. Anyone who can use the alter self spell, to convert them into creatures that can swim and breathe underwater (or fly over the water), can be a useful character in this campaign.
For instance, if the character is a humanoid wizard, it could alter self into an aquatic elf (to swim between the islands) or an avariel elf (to fly between the islands).
The important thing to remember is that to go between the islands you don't need to swim, and in some cases it may be safer if you don't swim. And while you are on the island, you want all the benefits of a land based creature.
If you want a good underwater caster(or excellent underwater support), then the witch with the Water Lung hex is an excellent one as it can be used continuously. It does have the drawback of eating most of a witch's rounds though(if used to support a party)
Water Lung (Su): An air-breathing target can breathe water or an aquatic target can breathe air. This lasts 1 minute. If the witch uses this hex on herself, she can maintain it while she sleeps, allowing her to safely sleep underwater.
As to summoner, an Eidolon can serve as an excellent water-mount as it can be granted a swim speed, gills and ability to be rode at first level. Or if you want, just go with the Aquatic base model for the Eidolon.