Complex sentence: It's the idea of this person being in the water--how they've got there, we don't know, but the idea is that they've been on a ship and they've been washed over the side so they're alone in this water.
Split into simpler sentences: It's the idea of this person being in the water, how they've got there, we don't know. But the idea is that they've been on a ship and they've been washed over the side so they're alone in this water.