Oh goodness, I'm getting the exact same issue as well.

I'm also having this issue. I have a fishing boat, 2 colonists, 2 dogs, and a few miscellaneous items. They sailed along the coast to a colony, left the boat, went to a settlement 2 tiles inland, then trekked back, only to get a 'Not enough space to house all pawn' message. Which isn't good, because I'm using an island mod, so without the boat, there's actually no way to get them back to the colony. I got rid of weight while visiting the settlement, I didn't gain any, so I'm not sure why this isn't working. Advice would be awesome! Even if the boat is a loss, does anyone know a way to get my colonists back in the colony?

I think dogs are counted towards pawn limit