Corrected entry: When Em is lying on the rock it is completely blood free. Later in the movie, we see the sacrifice that would have obviously left the rock blood-stained, though it was perfectly clean.
Corrected entry: In the scene where they are on the other island that they swam across to, in the back ground where they're making out you can see the statue of "god" behind them.
Correction: They did not go to "the other island", they just went swimming in the lagoon. I'm very sure if they were on another Island, you would not see the "God" statue. Not to mention the scene continues with them on their island.
Correction: On a tropical island, violent rain can clean even the most dirty rock.