I've learned if you park your boat too close to the shore, then monsters will come and attack it.. so, even though swimming (early game) is a possible death sentence.. you still have to park your boat about 20m from shore.. The Karve is about 20m in length (as an estimate). Some monsters will still throw rocks at the boat but they get bored of that after a bit. Also don't park it in shallow water. Make it so you had to swim a ways to get to shore, so monsters too will have to swim to get to it. Just something to consider. I have both my Karve and my Long Ship parked about 30m from my house. Only reason they are so far is I have my swimming skill up to about level 40.. (Swimming: Another thing you'll want to level up if you plan on having and keeping boats).