It's so annoying because the Terrapin's hitbox is entirely inside the chest boat's, so you need to break the chest boat to get the Terrapin out instead of just killing the Terrapin. Also, why would they need to get in a boat? They can swim! They can't even move the boat, and chest boats only have 1 seat, so you can't even move them with this! Also, the items in the chest boat go everywhere when you break it to get the Terrapin out. To be fair, you can probably use Splash Potions of Harming or something like that. I've been trying to lure Drowneds nearby so I can hit the Drowneds and deal damage to the Terrapin through the Boat with the Sweep Attack that comes from hitting the Drowneds with a Sword, but it's really annoying to do that.