The idea was simple:  Larger ships don't rock that fast (or much), hence their size.
That is why the default "Float" script just doesn't do it for all (mod- ) objects.

So here are 2 scripts: 
One that lets a ship rock gently from side to side, and another back and forth.

"FloatBig1" is X-axis (rolls slowly back and forth, this script is made for ships out in the ocean)

"FloatBig2" is Y-axis (gently rolling a tiny little bit from side to side, made for docked ships)