and yet that myth keeps going...
30% is the amount lost for an outboard jet. Those things use a different type of pump, and yes, are horrendously inefficient.
A modern seadoo/yamaha jet will have about a 10% hit in mpg's compared to a comparable in performance I/O. In some cases, especially at faster speeds, the mpg's are going to be nearly identical. Don't believe me? Go look up boattest.com. (if you search on my username and boattest I've posted some links between a yamaha jet boat and a glastron, the results were very, very close.)
I currently own 2 propped craft, and 3 jet powered, so i'm pretty much impartial either way. (as well as having owned 2 previous jet boats, and many, many prop boats) Jet boats have their advantages and disadvantages, but to absolutely discount one based on urban legend is pretty dumb...
I drive in water 6 inches deep A LOT. I'd like to see an I/O do that!