I recently asked Customer service at Rule (also Jabsco) this same question and here is their answer:
"If you wanted to remove as much water as possible, you could try installing a diaphragm style pump. The diaphragm pumps have internal check valves and are self priming to at least 6'. The only drawback is that the diaphragm pumps do not have as much flow as the centrifugal pumps. If you mounted the centrifugal pump switch higher than the switch for the diaphragm pump, the diaphragm pump could be used for the daily water seepage and the centrifugal pump could be used for emergency pumping.
A new pump that may fit your needs is the Jabsco 50880-1000. It's a diaphragm pump that doesn't need a filter made is made for bilge applications. The key issue is that the 50880-1000 has 3/4" ports instead of the 1 1/8" ports. Check out the link: http://www.jabsco.com/files/50880_sh..._pump_data.pdf
Looks like the pump for the job! The only drawbacks I see are:
1. switching the pump to be automatic and yet sense water levels low enough to keep the bilge dry. I'm wondering if I could turn one of those switchs that uses water to complete the circuit upside down (to sense water at a lower level) to switch the pump?
2. At over $200 (without switch) it's pricey!
Still, if I could overcome 1. I would bite the bullet on 2. If anyone has any Ideas, please let me know.
I think one of the major advantages to this pump is that the intake is on a hose and can be placed where pumps might not fit. Also, the intake is about an 1/8th of an inch off the bottom of the bilge and will therefore collect more water. All theory at this point, of course!