The crabs did surprise me, but it makes the actual area problematic. It doesn't really serve any purpose being where it is. Now I think it should stay on the island, but it needs to be toned down. Keep it a bigger challenge than the Sahuagins, but keep it relevant to characters who are still getting good XP from the Sahuagins or just moving on.
There's a quite a leap in the challenge rating between both areas, meaning anyone on that island not just gathering berries is probably gonna get destroyed by those 22 AB, fast moving 18 wheelers--I mean crabs. Anyone with the appropriate level to tackle on those crabs... well, why would they waste time running through the island to get to that cave?
It'd make the island relevant for more than the quest if you have a leveling path there, but right now you have the island, suitable for level 9-11ish and you have those crabs which are several levels ahead and still give full XP (and a scare) to a level 16 (15 with ECL), leaving a big ol' gap being filled by dead level 10s.