Its a known bug that I have reported a two times with other existing videos. Before the issue was reported as using Speed 2 or Depth Strider, you need to give more information.

Its not as simple as “PvP in water” There are an insane amount of variables happening, you need to figure out SPECIFICALLY what is causing it.

1. Your Armour, does it have depth strider? What else does your Armour have on it?
2. It is something in a beacon that’s causing it?
3. You have an insane amount of potions on you, is it when a potion is used under water?
4. What enchants do your weapons have, does it trigger when you use a specific weapon under water with a specific enchant?
5. Is it a combination of things from the above.
6. Both times I have reported this they have both been shut down for lack of information

What I need someone to do is find out EXACTLY what is causing it from the bullet points above, and then I can turn on the debug mode for the anti-cheat and you re-produce it, I will film it WITH the debug logs of it happening so we can send over a full proper debug log.