Bill-- An in-line water separator is an excellent idea, which reminds me that I need to change the cartridge before heading out to Neah Bay myself.

The size isn't that large-- about the size of an oil filter on my wife's Toyota-- but the mounting bracket does take up space. I've got mine on the inside wall of the transom with barely enough room to drop the cartridge.

Once you replace the cartridge and see the gunk that comes out, you will never have a boat without one.

While I haven't seen them, there may well be versions made for use with portable tanks that you could install in the fuel line. I'd probably check the catalog at a West Marine store.

Keith