Uh -- no. the OP posts pretty regularly.

I don't get it.

You fly out on a roundtrip ticket and sail back. You would then have to sail out to use the air return to get back. That would work.

Or you would have to get a second airline ticket outbound just to use the return leg of the first ticket. You'd have the same issue with that second ticket that you have with the first: buying a one-way or not using the return on a roundtrip.