Ive tryed everything and its hard can someone help me? And make it regen. Do you use cframe?
If you haven't already tried to use BodyForce then try. Just insert it into a part of the ship when you want it to sink and set the y force to some form of the ships -(SinkSpeed)*TotalMass. You can get TotalMass from the following:
local TotalMass=0 for i, v in pairs(ship:GetChildren()) TotalMass=v:GetMass()+TotalMass end