In the Sandship, I have gained access to the captain's quarters in the stern of the ship. However I can't access the room with the Big Chest in it. The chest and the room are blocked off by bars.
If I return to the past, one of the rooms (the rightmost one) is blocked by electricity beams. If I return to the present and then I can enter that room... and then I can stand on the switch to open the trap door to the room with the big chest in it. I shot an arrow through the trap door and opened one set of bars, but I can't progress beyond that.
How do I reach the Big Chest in the Sandship?