The boat materials can despawn or just be too deep to pick up. If you are setting up a base to deal with bonemass you really don't want to get anywhere near the plains. A longboat really can take quite a few hits... did you not have a bow to shoot the sea serpent with?

If you could not deal with the sea serpent in a long boat I think the last thing you want to do is keep feeding him while you get killed going there in a karve.

Make sure to mark each death location as they will vanish off your map when you die again... but honestly I think you would be better off remaking your gear and getting the stuff later than risk a lot of skill loss.