Oooh, Shiny!

What kind of plumbing fixtures are on your boat? There is more than an even chance that this is an area that the factory skimped on. Modern single-handle controls are not very expensive, especially considering that you will be buying only one or two. Why not upgrade?

The size and spacing of the holes for faucets are standardized - it is very likely that a standard single handle control will drop right in where your two single handles are (assuming that is what you have). 

The only hard part is making up the connections under the sink. But we are all boat owners here, right?  So I know that you have a couple of extra joints in your forearms.