I was asking myself the same question Archdeacon Monseigneur. Now that the Sea Serpent Trophy is required for Heavy fishing bait, I have been trying to get one. I checked out all my map icon locations, and finally after 2 weeks finally chanced upon 1. No trophy of course, they are extremely rare in my experience, after over 4000 hours of play I have only ever got 1. I have never had so much problem farming Sea Serpents. Prior to the Mistlands update, I farmed them over a couple of days obtaining enough to make 3 full stacks of Serpent Stew for my son, nephew and myself.

PS: Gulping down stamina potion if one gets low on stamina while reeling in either Sea Serpents or fish works to regenerate low stamina. I always eat Blood Pudding, Bread & Eyescream while fishing or hunting serpents, and have had to gulp stamina potion a few times, especially while reeling in lv 4 or 5 Trollfish. I keep my distance to 16 metres, but if I see a large lv fish coming for my bait I will reel it in to 10 metres before it takes the bait. Fishing skill is currently at 30.