Since I have searched all the coastlines and have not been able to reach the Lost Sea or the Enchanted Isle, I'm guessing I need to give that old pirate his Longtooth Coat and get his secret (treasure map?). Problem is that I haven't run across any on Skull Island. Any tips, hints, or spoilers?

You have to go thru Anephas' Shrine to get to the Lost Sea.

Silt

Must be a rare species. The only place I have seen a Longtooth is in Anephas's Shrine. I might have seen one on Skull Island, I'm not to positive about it though. If you check the walkthroughs on the W&W Homepage, on this site, you'll find that his treasure is somewhere near the Sunken City. You have to have those Conch Shells first, to get the underwater gear.

You can find longtooths anywhere, its just the luck of the draw, i had 2 of them spawn behind me when i was actually talking to Talrik himself , i've seen them on skull isle and the enchanted isle too, just about anywhere. if you don't have your spawn rate up to max, then do so as it will help you out too.

I agree with Shinobi - got them near Talrik the first time & then all the way into the pyramid (those bloody furs are worth only 14 gold though). As Shinobi says, put the spawn rate up, I always run at max spawn & max diff. [Just a little spoiler: the treasure is more or less rubbish & it is east of collassium].
Good luck hunting.