I guess I can imagine water in a thru hull that’s a few inches below the waterline getting cracked as the water freezes and expands, but I’ve never heard of that happening. But storing in the water isn’t very common in Maine so my not hearing about it probably isn’t very meaningful. Too bad insurance companies don’t seem to publicize statistics on these types of things.Assuming these are a foot, maybe more, below the waterline, I'm not sure they are the risk. It's squeezing or carving into the hull or expanding inside waterline thru-hulls that don't have anti-freeze past the closed ball, if any.
The place I stored in Maine is at the mouth of the Passagassawakeag River but the water in the harbor is salty enough so it rarely even gets skinned over and with 10’ tides it quickly gets broken up as the edges settle onto uneven ledges and shoreline and then pushed out of the harbor by the tides and current. But about 30 miles up the Penobscot River I’ve seen multilayered, 10’ thick chunks of ice breaking off from the shoreline and floating downstream. So my point is that the climate is only a small part of how much ice will be in a particular body of water, thus we’re talking about 2 harbors one 500 miles north of the other, and it’s the more southerly one that occasionally freezes over thick enough to walk on. So it seems that local knowledge of typical conditions for each harbor is more valuable than any other source.