Boat was on the edge of a water stream above sealevel. I dug 4 blocks out under the boat and the boat sank down 2 levels, and disappeared into the sand. Then I dug out 4 blocks of sand to uncover the boat to take this picture.
Perhaps related to https://bugs.mojang.com/browse/MCPE-32489, but that was fixed already.
The described behaviour cannot be reproduced on 1.15 Pre-release 2.