Karve survives Serpents decently enough. If you're fast enough you can set your sail path to intersect your floating grave then just stand up in the boat while the sails are up, and grab it while you pass it. Even if the Serpent follows you, downwind with the Karve you should be able to escape it if you head towards land immediately - Karve takes quite a few hits before going down. The serpent will follow you pretty close to the land but will generally just swim away once you park the boat and jump out - and if it doesn't or gets kind of stuck on the land, you may even get a free serpent kill if you're tactical.

Also, ALWAYS carry 10 wood with you so you can put down a bench as soon as you exit the boat and get onto shore. That way, you can immediately repair your boat anywhere there's land, then when you leave again, just destroy the workbench for the 10 wood and carry on.