If you stand on a boat that is on ice (possibly any surface and maybe the ice just makes it more visible) and you sneak to the edge of the boat (i think the boat has to be moving a tiny bit in order for it to allow you to sneak on the boat, i'm not really sure why sometimes you sneak off the boat and sometimes all of your  momentum transfers to the boat?), the boat will accelerate in the direction that you are moving.

I am pretty sure this is any boat but it happened to me with a bamboo raft with chest specifically.

This might be a side effect of MC-156980. Probably not because that's only when the boat's on water not ice

ok this is actually kind of rare and i was having trouble reproducing but if you put half slabs half a block above the top of the ice and one block over (like shown in the image) it is easier to reproduce but still kind of rare