With friends to play with, it's not so bad when you have multiple players with bows shooting at the sea creature. You can fight it off that way by yourself also but it's much more difficult, obviously, but it can be done. I'd honestly rather take on a sea monster than a pack of wolves lol. But anyway, try to stay out of open seas if you don't want to cross paths with it, and stick to the shorelines and shortest routes to the other islands (still doesn't guarantee a sea monster won't show up) to at least diminish the chances of an attack.