In my case peter, I have only crane load acting on deck, and hydrostatic pressure acting up ,the difference of both will create buoyancy, as well as bending longitudinally, so do you think, fixed support on both ends of ship is correct?

That's true. I totally forgot to remember, now the model which I took, its not complete upto bottom baseline.so in current case I fixed bottom.
I attach photo of one of very nice example(when to fix edge, when to fix complete face)
Please if you can tell, how shell, beam and solid, I should fix, like when to fix their edge or when to fix complete face. And do fixing only edge or otherwise only face, change dof of constraint which we applied for particular application?