you can make a U shaped terrain wall UP and far enough OUT that swimming in is highly discouraged. Once in a while something gets in, but they really have to work at it. The terrain is sloped so the boat floats normally, barely, but I can walk on it when the waves roll out if I need to recover nails etc. It supports the longship, just happen to have the karve today. Its in progress, this is the 'basement' and boating area of a planned home on top of it (the floor tiles behind me are where the home will go)

https://steamcommunity.com/sharedfiles/filedetails/?id=2505240586

https://steamcommunity.com/sharedfiles/filedetails/?id=2505240926