I smother all my connections in dielectric grease. My kayak is stored outside, too. (in the shade, on saw horses, under a tarp) Dielectric grease can be found at any hardware store. I think Andy carries it in SSB, I would image they do down south too. WD-40 works well, but can aide in the rusting process if you leave too much on there.
If you are good with a soldering iron, I say fix the plug and move on. No need to buy a whole new ducer or head unit if you can buy a plug and fix the issue yourself! Get her fixed, and back out there!
Edit- Or take a free ducer from YakDout. Nice. ^^^^