Sheldon, you seen transoms compromised like that before? My rookie mind tells me there would be plenty of room for ice to expand other than out, unless the water migrated into the transom area from hydrostatic pressure, but arent most welded on the inside? 
Now your bilge, livewell pump etc could have been, and the connections back there would concern me going forward.
Damage has been done at this point, on the warmer days i might opt for some thick mil plastic to cover the boat and get the greenhouse effect happening to melt faster with plug out.
yes,I have seen transoms that have failed from heavy ice in the boat,the problem starts with the wood in the transom,it soaks up the water before it freezes,when it turns to ice it acts like a jack and starts pushing things apart.
freeze thaw cycles make the problem worse,water gets into the expanded areas and when it freezes back up it jacks the parts apart even further and repeats itself with each cycle.
one thing to remember as well Nick,water weighs around eight pounds a gallon,if you were to have even fifty gallons of water in the back of a small boat thats four hundred pounds that expands in area when it freezes.
that is a lot area to take up the difference in size and it has to go somewhere,up and out in all directions and nothing can stop it.
you could argue that the ice will follow the shape of the boat and not cause any damage but there are too many places where that ice can get a toe hold and cause problems,think of where that water is sitting in every nook and cranny before it freezes.
also think of the wedging action between the transom and bench ahead of it,that is a longitudinal wedging action,welded hull or not,something has to give.
it may be years before it shows up on some boats but by that time it was forgotten that the boat had been filled with ice,hope that makes sense.
