The scroll that's in a cave by a beach where you've already found a scroll is the "Gem Grotto". If you're in the Hobbe Cave, backtrack till you come out of the entrance. Turn around, get in the water, and swim west (left) till you see a land mass in front of you. There's a sandy beach you can climb up onto, and then a sandy path back to a cave. The scroll's in there. Happy diggin'!