I have one of these that LL Bean used to sell.  This one is better.  Mine has spoke wheels and inflatable tires.  Aside from the wheels they are identical.  It folds up nicely so you can stow it pretty much anywhere, even on a kayak.  It rolls easily over grass, dirt, and even sand.  It has a kick stand, making it easy to lift the boat onto it.  The padded tubes can fold, as can the wheel assembly.  It's powder coated, though some reviews said it started rusting quickly.  The only rust on mine, which I got in 2008 is the metal wheels.  It's sold by Seattle Sports. Cost 150 dollars which includes shipping.

 

Looks like a nice dolly