Minecraft: 1.12.2
Forge: 14.23.5.2847

Mods:
CTM-MC1.12.2-1.0.0.29
Forgelin-1.8.4
ImprovedMobs-1.12.2-1.5.1
jei_1.12.2-4.15.0.291
Patchouli-1.0-20
TenshiLib-1.12.2-1.2.1
twilightforest-1.12.2-3.9.984-universal

Crash Report:
https://pastebin.com/45hvYNXL

Judging by the crash report I think it might be related on those guardian mobs that spawn when mobs are in water to chase you down and I assume a potion effect doesn't seem to agree with it. The passenger is a Cave Troll from Twilight Forest.

Note:
This was from a world with quite a number of mods added but removing most of them and having just the ones added and I've gotten the same crash to occur. Thought I was going to get a different one due to a corrupt world but the crash is the same one as the other one, just with the mods narrow'd down. Of course... I made a back-up of my world first before doing this.

I'll try to see if I could get it to occur again in a new world.

EDIT
Got it to occur again with a Zombie as the Guardian Boat's Passenger, apparently the Poison Potion Effect on the Guardian Boat is what's causing the crash to occur. The moment the poison damages it, crash. Also... since the poison will keep damaging it... the game will continue to crash as long as effect remains on the Guardian Boat. Unsure if just any sort of Potion Effect that deals damage will cause a crash, such as Withering, Radiation, etc.

Crash Report:
https://pastebin.com/6ZAT25b0