I'd go even further... a properly designed airboat can be faster than any water prop driven boat... if you design it in such a way that it leaves the surface and in fact becomes a plane when it's cruising... this is what makes "hydroplanes" so fast too... minimise contact with the water...

Have you ever heard of vehicles that travel "in ground effect"? If not, do a web search, there's plenty of info out there.

Try this for starters:[link=http://www.du-groundeffect.com/]Designs Unlimited Ground Effect Vehicles[/link]

As for high pitch props... they can be had at any major hobby distributor, most hobby shops, ...
Check out [link=http://www.apcprop.com/]APC propellers[/link]