Re.: "the boats dont have any weight"
...play around with the "MASS" setting in the minitweaker for example, it works with sh4, fun giving a GATO a mass of 2450 (OR 35000, you'll dent the seafloor 1.2 secs after loading the game)...try it
Problem is, if any MASS value is entered the boat becomes unbalanced regarding trim and floatation so it'd have to be rebalanced. A GATO set to MASS 1700 will sail along nicely until you do a crash dive and it starts to "recover slowly" somewhere around 260ft or something like that