In order to recreate this, place 3 blocks down laterally, place a boat in the middle and destory the block in the middle, then get in the boat, then exit it, it is important that you should stand on top of the boat before you then get back in. The server will then kick you for flying

Times it doesn't happen:

• when you just get into the floating boat
• if you only walk on top of it and get in 
• when the boat is on the ground

Attached is an image of it. A simple fix would be to just enable flying but who wants cheaters in my singleplayer multiplayer server aha. I realise the title is misleading, you first need to get in and stand on top then it will kick you...

https://www.youtube.com/watch?v=wVaF0wMMJ-s

^This also causes a kick for me