Boats occasionally fall through the ground either when placing the boat down or while getting out. When this occurs while getting out of the boat, it eventually comes back up the the surface on its own, but when placing, player has to break the ground to pick the boat back up.

I play on a realm that has an ice boat road in the nether and everyone who uses it has had their boats sink under the ice frequently, but we cannot find a common cause to replicate the issue as it seems completely random.