I picked up one of these http://www.defender.com/product.jsp?path=-1|51|299222|315197&id=105241 last year and installed it this Spring. It sits on top of the tank. Works as expected, and I don't expect to have another issue so long as I own this boat.

A diaphragm pump is the right approach. The traditional macerator pump that uses an impeller is just wrong for the application, IMHO. You can't run it dry without damage, yet the whole purpose of the pump is to empty the tank. So if it's installed where you can't hear it well and it runs too long it's toast. Then you have a messy job on your hands and no overboard pumpout capability.