I would love to hear from someone who has played an aquatic campaign. What follows is simply my observations about what upsides the rules offer:

3D Tactics

• While this was listed as a downside because of the hassle, it is also an upside because it opens up new tactical decisions. A quick thought experiment would be to imagine a standard game where all PCs and most enemies have the ability to fly.
• Structures can assume 3D movement, which opens up new designs, albeit at the expense of pit traps.

Melee Heavy

• Because ranged weapons are restricted, closing to melee will likely happen even more than in a standard game. Because the majority of statted monsters are best at melee, this adds to the challenge, letting the monsters make use of their potent abilities more frequently.

Alien World

• While the relative lack of source material is a downside, the world is wide open for home-brew creatures, especially incredibly large creatures and horrors from the deep.
• Even a "standard" race like the merfolk are alien. Do they lay eggs like fish? Probably. Also, they likely eat their fish raw and don't bother with much farming or cooking.
• You can take the large treasury of monster stats out there, add the aquatic subtype and a swim speed, and then re-skin them to make an old baddie feel brand new.

I would love to hear from someone who has played an aquatic campaign. What follows is simply my observations about what upsides the rules offer:

3D Tactics

• While this was listed as a downside because of the hassle, it is also an upside because it opens up new tactical decisions. A quick thought experiment would be to imagine a standard game where all PCs and most enemies have the ability to fly.
• Structures can assume 3D movement, which opens up new designs, albeit at the expense of pit traps. Perhaps the paradigm is inner-vs-outer rather than higher-vs-lower.
• Depth strata can provide means of retreat. Various species should be able to go to great depths without taking pressure damage (1d6 per minute for every 100 feet below the surface).

Melee Heavy

• Because ranged weapons are restricted, closing to melee will likely happen even more than in a standard game. Because the majority of statted monsters are best at melee, this adds to the challenge, letting the monsters make use of their potent abilities more frequently.

• Thrown or dropped weapons will not simply sit on the ground, waiting to be picked up at the end of combat. Unless you're fighting at the depth where your weapons have neutral buoyancy, they will sink out of reach or possibly float up toward the surface. The same goes for corpses and loot.

Alien World

• While the relative lack of source material is a downside, the world is wide open for home-brew creatures, especially incredibly large creatures and horrors from the deep.
• Even a "standard" race like the merfolk are alien. Do they lay eggs like fish? Probably. Also, they likely eat their fish raw and don't bother with cooking. The farming they do is probably very different, involving cubical or spherical pens for edible fish/crustaceans/etc. They may have domesticated their own plant species that they tend near the surface.
• Because pressure differentials can be lethal, the depths at which a given species are at home can be a big differentiator. Perhaps merfolk have no special ability to dive deep, and so stay near the surface. If Kuo-toa are really Deep Ones, then perhaps they should have the special ability to be immune to damage from pressure, which suddenly changes the dynamic between merkfolk and Kuo-toa.
• Most of the probably playable races can breathe air, but there should really be fully Aquatic races that cannot breathe air any more than humans breathe water. Creatures and races that are at home at deeper depths should suffer when close to the surface.
• You can take the large treasury of monster stats out there, add the aquatic subtype and a swim speed, and then re-skin them to make an old baddie feel brand new.