Possible Clues:

• No paying guest has this room above water
• Nothing to pay for one's keep over the water
• A shipping term for complimentary meals?
• Nothing to pay for your keep above the waterline?
• Nothing to pay for keep above the water line
• Nothing to pay for being kept over the water
• Nothing to pay for one's keep above the water line
• No charge for meals during term at sea!
• Space above waterline offered to sailor working his passage?
• Nothing to pay for your keep on the ship so long as you're so high above the water