Dave,
Check out a FloJet. Model 4125-114. Five gallons per minute, 12 V and comes with a debris screen. This pump can be ran dry without damage. A seasoned Amphicar owner recommended this model and I am on my third year with no troubles. I also installed a back up pump under the back seat. As I recall I spent about 40 or 50 bucks on that one but the FloJet was about $180.00.
Whatever you do, don't go cheap on your primary bilge pump!
Brian Crombie
Faribault, Minnesota