Methinks "Prepare to Fight" = Build Templates.
Off the top of my head...
No More Aquabreathers.
The game works just fine without it having to be an item with a specific slot, it can just be one of those 'gamey' things that you start the game with and don't have to worry about. Alternatively, Aquabreathers could remain an item but wouldn't overwrite the helm slot. Different aquabreathers could give passive effects when underwater, to help make underwater combat a little more palatable. For example, there could be Aquabreathers that gives increased swim speed, increased endurance regen, create a protective bubble around you when downed, etc.
Surface
When you reach the surface, you lose all of your skills and abilities. This is especially annoying when you're trying to rally. Treading water shouldn't completely negate your underwater abilities, they should just only have an effect under the surface (i.e. if you go to use a ground targeted skill on the surface, the area of effect uses the surface as the 'top' and it is projected downwards).
Area of Effect Indicators
They should be cylindrical and 'soft snap' onto enemies (i.e. if there's an enemy below you, it would be difficult to move the AoE down because trying to aim it that way only brings it closer to you). When you place the targeting reticule over an enemy, the AoE would simply 'stick' to them unless you moved it away.
Underwater Skills
Basically, a while pass would need to be done where every profession has at least one elite skill that functions underwater. The ideal method would be to give every
race an underwater skill so that players have at least one by default (unlock it for free for best effect). There should also be an "underwater build" tab so that you can adjust your underwater utilities at any time.
Increased Awareness
If possible while underwater, the field of view should be pulled back a bit to give you more room to see around your player (similar to how bosses work). Too often you're maneuvering in three dimensions and bump into things. Additionally, underwater ranged attacks need to leave longer-lasting 'vapor trails' underwater so that the direction you're being attacked from becomes more immediately obvious.
Probably a dozen little things that could help too, but those are the only ones have at 8am.