Stoves for boats are silly expensive. Not only do they cost more than they should, most of them don't work all that well either.
Here's a better and more affordable than most single burner alcohol stove...
While this beer can stove costs pretty much zip.
How To Turn A Beer Can Into The Only Camping Stove You'll Ever Need from Tom Allen on Vimeo.
Of course, you're going to want to come up with a means to adapt one to the rigors of being on a boat but that's hardly rocket science is it?