JimBob, not a bad idea, but not one that would be particularly simple to keep up with. I go to work in the morning and the gas is at one price, and has changed in the evening. And I don't know how far some folks would go in a boat to save .03 to .10 cents.
Of course you could develop your own website...
Bottom line, just like cars, if you want to use your boat, you'll fill it up and bite the bullet. And is it worth a 20-30 minute ride minimum to save a few cents per gallon. Again, not a bad idea, just not one meet with an abundance of enthusiasm.