Winkle?? Please help!

Okay, so all I have left to do with the other estranged sims is to give Dr.Feelgood the spicy green curry but I'm having an issue with making the curry. I have the eggplant, the chili, and the coconuts but I can't get the sugar. In order to get the sugar seeds from Ms.Weeder I have to trade her a watermelon...which I don't have. I've gotten a couple when they washed up on shore, but there haven't been any washing up in about 20 game days. And in order to get watermelon seeds from Ms.Weeder I have to trade her turqoise coral and a winkle. I have the coral but I can't seem to find a winkle anywhere. It says it can be found by diving for it, but I've tried both near the boat and on the third island numerous times and still I can find no such shell. Does anybody know how I can find one? Or if there's any secret to getting a watermelon to wash up on shore?