There's a lot of overlap between ham radio and marine
radio. I don't know that anyone has jumped in to publish "Marine Radio for Dummies" yet, but maybe that's because there's already so much ham stuff out there.(G)
First off, if I may suggest, if the microphone is attached to the radio with a little screw collar, so the mic and cord can be unscrewed and removed? Take it off and stow it. This was traditionally done by radio operators to "secure" their station and prevent unauthorized usage, and as long as there's no mic attached to your radio, no one can give you flack for having it. Without a means of transmitting (no mic, no morse key) it is just a receiver and as such, the FCC says "Have nice day!" and no one else is entitled to comment.
The ARRL (ham radio national assn. in the US) actually has a book on marine/mobile setups, and one of their comments has long been that even for two "identical" boats, the same antenna will not always work the same way, because there are so many differences, like rigging
quality, joints, oxidation on connections, etc. between them. So your choices are to say "I want this kind of antenna because...and that's what I'm getting." Or, "I'm willing to experiment
over a couple of weeks or months and see what works for me." There's no magic solution for what is best.
Most use a backstay antenna of some sort because, well, the backstay IS there, and often works well enough.
But any kind of antenna really resonates best with one band and one band only, and if you want to do more, you may need to add an antenna "tuner" at the base of it. These really don't "tune" the antenna, they just fool your radio into thinking that the antenna isn't as bad as it is on other bands, which again beats nothing.
And you'll also find a lot of basics about getting enough power to the radio, running a counterpoise
(aka "ground") for it, and learning
how important simple wiring
connections on the radio's antenna wiring
can be. Again, if there's nothing around for marine
SSB...ask your local reference librarian about that, and ham radio, or contact the ARRL.ORG online. And don't forget to contact SEA, they are still in business and if you don't have a manual, I'd bet they can still find one for you.
SEA has a good reputation, odds are that if you get it set up nicely and understand what it can and can't do, you'll keep it, at least for a while.