You could alsways hire/beg for a merc :) I'll do it for the serpent meat ^_^

i've got an hour or two to spare, contact me if you desire my aid

I'll take nothing, i will bring myself and a long boat. The serpent will die. I purposely go on serpent hunts. frost arrows work miracles

Its a real bad idea use a raft for that
Are you sure that spawn at night ? If thats right most despawn at daytime ( to spawn again in night until die )
If just spawn at storm but in daytime that thing will keep in the zone until you kill it

Apparently serpents spawned due to weather will not despawn while serpents spawned due to nighttime will.

Since yours is still there it must have been spawned due to weather despite it being nighttime, and will stay there until/unless it gets killed.

Is it so far out that there is no land nearby you could use for archery action?

bad weather serpents usually stay alive once they got active. I dont know how difficult it is to reach that gravestone but maybe it would be better to bring a well skilled friend with you that kills the serpent with a bow.

Or you wait till you have a karve - you can drive over your gravestone and grab it without leaving the boat. And a karve can sustain several snake bites.