I love underwater combat, and a recent run of pl_underwater_rainy has given me a lot of practice with it. Here are some things I’ve learned about fighting in Davy Jones’ locker. (Also, apologies for the fuzzy pictures; it turns out TF2 Replay doesn’t like photos taken underwater.)
1) Understand which of your weapons don’t work underwater. This is pretty basic, but you don’t want to waste valuable time trying to fire your Huntsman or stun with your Sandman only to find out they don’t work. Rule of thumb, things that involve taunts don’t work, so no stun, no Holiday Punching, etc.
2) Use the spacebar to dodge. When underwater, the space bar makes you rise. This can give you the high ground, something useful in any fight ever, and its a direction the enemy is less likely to expect because it’s not usually an option on the ground.
3) Get good with a projectile weapon underwater. Normally, once underwater both classes switch to their most powerful available hitscan, since they operate the same as out of water. This means that if you learn how to use a Grenade Launcher underwater, they’ll be severely unprepared to fight that. The sticky launcher works pretty well too because charging them can counteract the reduced projectile speed underwater.
4) Explosive jumping has reduced everything (including self-damage). Explosive jumping underwater if you have the right arena can completely change the setup of the fight. Because you can’t be standing on the ground, you automatically take reduced self-damage but gain full knockback. Of course, you’re also fighting extreme water resistance but I don’t know of many bodies of water requiring that much to reach the top. This also adds to your height advantage, which is as useful a thing as it was in tip 2.
Final tip: If you’re good with an underwater shotgun, lure people to their deaths as Pyro by choosing underwater fights. I’ve had people leap into the water assuming I’m a free kill, only to receive six buckshots to the face for their effort.