Help! Quest : Pirate's Crops

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For the quest, Pirate's Crops, I cannot find the secret cave where the crops are located. In the log, it says the cave is marked by a cross and skull on an island in the Eastern Islands. I've been sailing around for ages and cannot find this.

Is the Underwater Cave the cave? Because I've explored long before getting this quest and found nothing. And right now, I dont see the crops anywhere in there but its the only cave (other than Lucky's cave/mine) found in the Eastern Islands.

Any help would be appreciated. Thanks!
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