cannot get my ship to stay on water

[patrick_pagal]

Anyone who can help! I'm doing an exercise for school and we have to simulate 4 ships rocking to the motion of the waves. They all need to move at the same time but different amounts. I thought I had this set-up right but when I push play the boats end up sinking or moving above the water. How can I get them to stay on the water but also have them rock different amounts simulating the wave motion.

My assignment is due on the 21st. I just came upon this site and would appreciate any help possible. I can upload what I have so far if it would be helpful. Just let me know

Thanks in advance!