What race is your fisher? What skills?
a. A half-trained blue beetle can kill normal sea serpents (deep sea serpents are a whole other matter).
b. A human with 0 magery can successfully read a blade spirits scroll 27% of the time, as can any other race with 20 magery.
c. If you can cast Mark directly (not using scroll), you have enough magery to use 8th circle summon scrolls like the elementals and Energy Vortex. No Eval Int is needed, and the summons usually make quick work of serpents.
d. another oddball option for a non-tamer is to have a tamer get you 4-5 ostards, and once they are bonded, have them attack the serpents. Characters have 4 stable slots now, more if a tamer, so you could theoretically have 5 ostards (one of which you'd always have out as a mount), or a giant beetle and 4 ostards (and either go with 4 ostards and an ethy, or 2 ostards and the beetle).
e. The Vollem - especially if you can get one of the ones that was offered originally in a crystal with the SA launch - is a reskinned, not-mountable, nightmare, so trained up they can be good for serpent killing. Even the golem versions are good, if you can keep them alive long enough to train up.