Serpent is indeed likely to be death to a raft, but by the time you reach a karve you should be able to take them on if you wanted. I wouldn't advise taking on multiple at a time in the karve but the chances of finding multiple at one time also aren't that high to begin with.

If you do decide to fight rather than flee, the scales can be nice for the shield but they sink in deep water and the shield it offers can't really parry so it has somewhat limited use but can be helpful around swamp-mountain tier. The meat however, floats, and makes some of the best food in the game so even without the harpoon I find it worth it to kill the angry snakes.

The harpoon is not necessarily an item you find, it is one you craft but it requires crafting materials from another seemingly rare ocean mob. However, you don't HAVE to have the harpoon to get the scales either. If you have a serpent spawn relatively close to a shoreline, especially a channel or inlet, you can kite it into the shallow water and let it snack on your boat while you finish it off, boom, scales and meat:
https://steamcommunity.com/sharedfiles/filedetails/?id=2622395014