Q, I'd suspect that the market for "offshore cruisers medicine" is 1000 sailors worldwide and you might sell your book to 100 of them before the market saturated. Add another couple of hundred interested people and if you are lucky, 200-500 per year, which means no publishing house can afford to print it and if you self-publish or demand-publish, that might bump the price to $50 and slow down sales even more. Which is not to say you shouldn't do it as a labor of love--but don't expect something targeted to that market to be profitable.
I have read books on offshore medicine, wilderness medicine, etc. and while I can't remember the name I know there are a couple of good ones out there oriented to a more general survivalist/wilderness audience. You might find some references at www.equipped.org
, Doug Ritter's site. I guess the big question is, do you want to encompass appendectomies and trepanning, or stop somewhat short of that.