Stranglekelp sells on the AH at 4 to 4.5 g for a stack of 20, and it sells FAST! The reason for this is that it's used in a ton of potions that people use to level their alchemy skills. For some reason, there is a perception that it is difficult to find and collect, but I've been collecting it on a daily basis in huge quanitites with relative ease.

All you need to do is go to Zoram Strand in Ashenvale (west side of Ashenvale) and swim out into the ocean a little. Start from Zoram Strand in the south and swim north, collecting Stranglekelp as you swim. See the map below: