There's a very simply answer to all of this.
Get a ham license to operate in the ham bands.
Get your HF license to operate on your boat in the marine frequencies.
(And you do this with your own country)
Then purchase a rig that is built for operations in both bands.
If you're trying to save money, then that's the best, cheapest and most legal way to do things. If you are in the US and break the laws they can, and will, fine you up to something like 10,000 US dollars for every breach of the law they find you have made.
By the way, in the US Hams CAN legally modify marine equipment to operate in the ham bands without any type acceptance or any of that stuff. In fact, hams can legally modify pretty much anything INTO the ham bands.
They can't modify it OUT of the ham bands.
Rick Donaldson, NØNJY
It's better to be hated for who you are, than to be loved for who you're not.
Let those winds of change blow over my head,
I'd rather die while I'm living than live while I'm dead - Jimmy Buffet