Spoilers included.
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Have you found the area to use the hook and line yet? If not, keep looking. The area to get to that is to the right from where you entered the Sea Caves and the passageway leads upwards. If you haven't found the hook and line yet, keep looking -- you'll need them.
If you've found that area, have you gotten hold of a set of spiked boots and a plank (the plank is found out on the beach)? Again, those are necessary to complete this area.
You should also at some point receive a key to Marten's tomb but the obvious door isn't the one you want. You want to search around those slippery areas with one person wearing the spiked boots. The entrance is past one of the areas and a chest is found past another.