It seems the followcamera crashes the player when removed from the world after leaving boats. This issue can happen with changing characters via the celestial portal and more notably happens with mods that allow you to despawn/change characters.
1. Have celestial portal 2. craft moonrock idols 3. craft boat 4. jump on and then off boat (my boat crashed and sank after I got off, no idea if that is needed to reproduce the crash) 5. Offer moonrock idol and attempt to change characters. 6. Client Crash.