I don't think they despawn. They will just be frozen/not loaded until you get back, where they will continue where they left off when you, or someone else, returns and loads the area.

I hope you didn't have anything important in that boat, but I think cargo floats. Nails sink though. Once the boat is toast they will either go back where they came from if it wasn't too far away, or get distracted and attack something else. If they got led too far from their spawn area they might stick around.