you can crouch whilst riding a boat with the emote feature

this works in at least 1.20, as far as i'm aware. who knows, maybe it works in 1.19.

anyways, when in a boat, if you use the quick emote menu, then exit the quick menu before pressing [toggle crouch], you will crouch while in the boat, and prevents you from leaving the boat or uncrouching. the only ways to leave are if you die or if the boat dies.