If any of the boats is placed on a slime block, a player who gets in it will get launched in the air. Once the player gets out of the boat, it will fall back down and break. How long the boat has sat on top of the slime block affects how far it will be launched when the player gets in it. If the boat launches more than 150 blocks and the player has not yet gotten out, the game will crash.
I can confirm this. Leaving the boat on a slime block for around 20 seconds will send it launching into the air. The game freezes, says "not responding" forever, and you have to close it.
Game Output when the launcher reopens says the following:
[20:39:05] [Server thread/WARN]: Boat (vehicle of MCEvan99) moved too quickly! 0.0,13.639999695122242,0.0
[20:39:05] [Server thread/WARN]: Boat (vehicle of MCEvan99) moved too quickly! 0.0,13.47999969869852,0.0
[20:39:05] [Server thread/WARN]: Boat (vehicle of MCEvan99) moved too quickly! 0.0,12.519999720156193,0.0
[20:39:06] [Server thread/WARN]: Boat (vehicle of MCEvan99) moved too quickly! 0.0,10.99999975413084,0.0
[20:39:09] [Server thread/WARN]: MCEvan99 was kicked for floating a vehicle too long!
[20:39:09] [Server thread/INFO]: MCEvan99 lost connection: TextComponent{text='Flying is not enabled on this server', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[20:39:09] [Server thread/INFO]: MCEvan99 left the game
[20:39:09] [Server thread/INFO]: Stopping singleplayer server as player logged out
[20:39:09] [Server thread/INFO]: Stopping server
[20:39:09] [Server thread/INFO]: Saving players
[20:39:09] [Server thread/INFO]: Saving worlds