Crash Report:
https://pastebin.com/pYskjs1F

Minecraft: 1.12.2
Forge: 14.23.5.2847

Mods: 3
ImprovedMobs-1.12.2-1.6.1
jei_1.12.2-4.15.0.291
TenshiLib-1.12.2-1.2.3

If a Guardian Boat spawns on certain mobs, such as a Wolf... the game will crash to the point where the world will get stuck in an endless crash loop. The only way to fix this is to either delete the offending entity or remove the Improved Mobs mod.

It's not only vanilla mobs but some mobs from other mods can cause this to occur as well... to name one... the Betweenland's Angler Mob, and while there is a Blacklist/Whitelist option... it may be a long list to work with as you won't know which mobs will cause the crash to occur until it's too late which will then require removing the mod, deleting the offending entity, etc.

If possible... could there be a config option to disable the Guardian Boat regardless of the Blacklist/Whitelist?