1) If you target the boat deed when attempting to place a new boat, the new boat will be created somewhere around 100 100 -5, and the player moves there along with it.
2) It's possible to place a boat directly on top of an existing boat.
I think this also goes for houses. I wound up in the middle of the ocean with a castle. But, the same placement code is probably used for boats, so this fix will probably solve the house placement issue also.