Exiting a boat with a block directly above and a block directly below will cause the player to fall through the floor. If there is only one block below them they will fall completely through it, but if there are multiple they will become stuck in the block directly below. Bug occurs in any gamemode, in singleplayer and in multiplayer.
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This is a duplicate of MC-148869.
This will be fixed in 1.16, see MC-148869