This might not be the answer you are looking for but you could loop through every part in the character’s body and change it’s density to a lower value?
You could get rid of the Terrain water and use a CanCollide false Part or Mesh so it doesn’t have buoyancy.
If you wanted to use Terrain Water and walk on the bottom you’d need to cancel out the ‘swimming’ animations as well. You may be able to weld a CanCollide false Part with increased Density to help out, but this may affect how the player moves since I know that Tools that aren’t Massless can cause issues.
How would I do this? I have just looked around a bit on the DevForm and on the Roblox documentation and found nothing. I also can’t find the property “Density” under any of the character’s body parts. Could you perhaps link a tutorial/article on density? Sorry for the inconvenience, and thanks for the help.
Check out this information BasePart | Roblox Creator Documentation, but in the Part’s Properties when you select it you should see a checkbox with CustomPhysicalProperties already in it.