Hi Dave
Agree the image of a CMP floating just doesn't make sense. Which made me do a little thinking.
Lets say we are talking about a C60L with the steel cargo body Curb weight 9,700 pounds. The rear body is approximately 90" wide 144" long and has 30" sides. Now my math says that would be about 225 cubic feet of space. Water weighs 62.30 pounds per cubic foot so that would work out to the rear body initially displacing 14,000 lbs of water. Would of course depend on did it float relatively level. But it would seem it would try and float until it leaked enough to sink or the water just poured over the side because of angle.
So if you drove/dropped a reasonably well sealed radio box truck into the water it would float.
Cheers Phil