Lot of negative energy going against water fights these days, and I just want to offer some advice and things I've learned from doing it to even things out a bit for people either just wanting to counter or to have some fun seal fights.
1. The complete kit is necessary, or you will be at a disadvantage. You need a wetsuit for speed, diving goggles for clear vision, a rebreather so you can stay underwater for periods of time without drowning, redgulls because they are necessary for swim speed, and an sdar with a good supply (reccomend 20) of 20 round dual purpose magazines (the only ones that work through water.)
Optional: A sidearm to switch out when you cross onto land, or an alternative weapon in the backpack smaller than 762.
2. SDAR has an EXTEME bullet drop that begins almost immediately, and a very slow movement speed. When leading a target, lead far far ahead, and when aiming for a target, less you're in dancing distance, shooting above them is necessary for headshots. Shooting at a target 40 meters away requires you to shoot almost 50% of their height above the target
Practice:
Go to a beach with a lot of sand and fire and wait to see where the shots actually land, and most will realize they drastically the bullets travel time and drop. 10 minutes of doing this will make you a far better water fighter than you were before. This effect appears to be increased in the water as well, meaning you have to aim a great deal over your target, not aiming at them, even when relatively close.
3. Using alt to pull your camera up to see an out of water target, and then dropping the camera back down and fully autoing the area they are in is an effective low risk tactic that requires a substantial amount of ammunition. (Chances are you will be inclined to aim too low though, as above, so practice).
Alternatively, using alt you lock your crosshairs onto a specific target and watching that target also works, but the recoil only makes this work for one round at a time.
4. Water refraction is a thing. When aiming down into a shark in deep water from outside the water is the only time to aim low. They aren't actually where they appear to be, just reflected to the top but directly underneath.
SHARK VS SHARK
5. Bullet drop and slow bullet speed making dodging bullets underwater a very practical thing to do. Moving in sporadic patterns and and drastically increasing or decreasing depths can turn a shark on shark battle into a 5 magazine event. If you are lucky enough to have a rock or some cover, use it, as most will be aiming too low anyways, it usually always works to catch the strays that might actually hit you.
6. If you don't have a wetsuit, just stay out of the water. You move way to slow and you'll never win an sdar fight with someone who can dodge better than you.