If you summon an End Crystal in a Boat, it will move to the front of the boat and prevent you from being able to ride the boat. All mobs will jump to the front of the boat if there isn't someone riding it, but most will move to the back if you get in.

There might be other Non-Mob entities that cause this also.

/summon boat ~ ~ ~ {Passengers:[{},{id:"end_crystal"}]}

/summon boat ~ ~ ~ {Passengers:[{id:"end_crystal"},{}]}