i wrote a bouyancy volume a while back that will apply an upwards force with variable bouyancy and you can find it in the
"Asset Library\Presets\Skyline Scene Types\Physics Volumes\Physics Bouyancy Volume.xPreset".
Position and scale to fit your water area

There is also a tech demo in:
"Asset Library\Scenes\Tech Demos\Physics\Physics_Bouyancy.xSkyScene"