You've obviously never see how heavy snow can get, especially if there's a rain storm or freezing rain storm along with it or if it is the wet concrete type New England snow. A couple of feet of the wet-concrete type snow we get in New England can add enough weight to a boat to bring it down a few inches.
If the cockpit drains get clogged and the cockpit fills up with snow and ice, that can be a pretty significant amount of weight as well. Add to that a hose or two bursting from ice freezing in it... then you've got a pretty good recipe for sinking a boat.
BTW, to give you an idea of what the problem is... roofs in New England are often designed for 40-70 lbs. per sq. ft. of snow load. Even with that kind of rating, we've had roofs failing due to the weight of snow and ice this winter. 3-5" of old snow weighs about 5-6 lbs. per sq. foot. If the snow was wet to begin with, that can be off by a factor of two....
Using the base figures 3" of snow weighing 6 lbs. per sq. ft., with snow that is 15" deep, as was the case this past week in much of New England. You're looking at 30 lbs. per sq. ft of boat area..... probably more in the cockpit, since snow would tend to build up there due to drifiting... A 30' boat is usually about 11' wide... figure that the area of the boat is about 60% of the area of the rectangle, due to the shape of the boat. 30*11*.6=198 sq. ft. 198 sq. ft. * 30 lbs. of snow per sq. ft. = 5940 lbs. of snow.
The PPI rating of a Catalina 30 is about 1000 lbs. per inch. That means that this much snow would sink the boat SIX INCHES. How many boats do you know of that would be safe if they were sunk an additional six inches without any additional preparation.
In southern California, sailboats are regularly sunk by sealions and seals wanting to sun themselves on the boats...
I just had to jump in here.
I find it extremely hard to believe that you could pile enough snow on top of a well found sailboat to cause it to sink. After all there is already several thousand pounds of lead/iron hanging under it.
If it sinks it's because there is another problem that the owner failed to address and just got away with it until times got bad.