The Legend of Zelda: Phantom Hourglass Requests

the first crest for the red door in the temple on mercay island.

Big Gold Rupee

I had the sea crest on my map, collected the key at the bottom of the ocean, but the green sea crest simple is now gone from my map! So of course I cannot complete the Ocean King's Temple quest. Can anyone remind me or tell me how to regain the sea crest so I make progress. Any help is appreciated. (I'm so tired of being thick about these things).

need help on ocoan king temple on the third time you go ther for the south eastern sea chart!!