you could also create a sinking / float action by
turning off gravity or making the object massless
when the object is beneath the
water hight (after sinking a bit), when its beneath some limit or for some time you then apply
a small upward force to the object , when the
object reaches above a certain hight you again
switch off gravity ...

this will also alow your object to rotate around
more freely like an outer space environment. .