You are limited to the receivers sensitivity. I don't think that radio was really designed to work with those types of signals. Usually those are for broadcast AM and FM. Making the antenna longer does help but only to a certain extent. The wire is not matched to the antenna input on the radio. Attaching the wire to the whip antenna will net the same results. If you are really serious about this project then you might want to invest in a communications receiver like an Alinco DX-R8.Yes they are expensive but the Sony does not compare to the larger receivers.
You might be able to add a shortwave antenna
tuner. That will help match the antenna to the antenna input impedance.
Now, let's fix that antenna of yours. You want to get it to the length that the antenna is resonant. Some might say it is a waste of time but I think any antenna that is "tuned" is better than a length of wire. So look at the frequency you are using. If this is the primary then use the formula 234/F
(MHz) for a 1/4 wave antenna, or 468/F
(MHz) or a half wave antenna. As an example...
234/13.2 = an antenna length of 17-3/4 feet. For the half wave just double, or 468/13.2 = an antenna length of 35-1/2 feet. This will at least get your antenna vibrating on the resonant frequency.
Lastly, if you are on the boat and your radio has a ground lug somewhere (or even make one), take a length of wire and attach a bolt to the wire. Throw the thing overboard, er... the wire with the bolt, not the radio. The bolt and wire should be submerged about 6 inches. This should help to eliminate some of the noise and it will act as part of the antenna also.
Lots to do, I know. Good luck!