sea Serpent trophies sink IIRCC.

You have to use an Abyssal Harpoon to pull them onto land or even just lure them into shallow water.

I've had the serpent trophy drop when I killed them on land so many times I started throwing them into the obliterator. One can only put so many serpent heads on one's fortress before it starts to look a little tacky :/

Good hunting o/