When you die in water, the gravestone is dropped and you lose all of your things. This is a known issue and a user named JosiahFu made a project that fixes it (https://github.com/JosiahFu/GravestonesFix), but it isn't included in this project.

We were playing in Minecraft Java 1.20.1 Fabric v0.92.2. I hope they can solve it and that this issue helps.

Upvoting this, I just had it happen to a server member ;-;