[A fellow user by the name of "christopher john" has also described the same issue for a previous version of Minecraft (1.14.3) and the topic was switched to "resolved" however the bug can still be recreated fairly easily]

Issue: After a player gets out of a boat in vanilla Minecraft, they can jump a few times and as soon as they land on a solid block (in this case, either side of the boat) they take damage equivalent to the amount of blocks they jumped and "fell" from despite only falling from a one block height, this bug is similar to the 'lead' item glitch where if a mob is dangled from a block of air and then let down onto a solid block, they take damage.

This isn't such a urgent game breaking bug but a mild inconvenience to anyone who decides to jump in a boat a few times.

PS: I can personally assure you this is a pure vanilla scenario and may not be able to be recreated on other versions/ modded versions of the game.