There is nothing under the ship but the rudder and the buoyancy blisters. If it helps any the ship alone masses 18.5 tonnes and the M1A1 is exactly 11 tonnes. So 29.5 tonnes. I do not have better buoyancy installed, but I have fixed my thrust reversal problems. Also the M1A1 relies on Kerbal Foundries for its tracks, if that may be the cause of any problems.