I checked the Bestiary and it says that the only sea snake that drops a clue is the level 149 giant one. I got sea snakes for slayer, and I was somewhat AFKing between the level 62 hatchlings and the level 90 young, and I noticed a clue on the ground. I'm definitely sure that I wasn't close enough to a 149 to kill it for a clue. I'm currently doing the trail to see what level it is.

Level 2 clue... Click on their names again and read the Miscellaneous drops carefully. All three sea snakes drop level 2 clues. http://tip.it/runescape/index.php?rs2monster=&orderby=&keywords=sea+snake&levels=All&race=0