Practical Sailor magazine is like the CU, but for boat equipment. Look them up as they have done bilge pump tests, many times over the years.
*All of the bilge pumps that you normally see are rated at zero head and 12.8V if i remember right. It's been a while since i really bothered to look since mine are in good shape but I think those ratings are in the mfgr. details. They certainly are explained in the Practical Sailor.
*Also most of them need to have output hoses that have no sharp 90o ftgs or the like and smooth walled ID hoses. Internal corrugations seriously impede water flow.
Add some lift to get the water out of the boat, and that is a given, or reduce the voltage as the batteries run down and the pump output will be seriously reduced.
*Do not take the brochures as gospel. Buy the biggest you feel is necessary with the realization that in your actual boat under less than ideal conditions the output may be lessened by 25 - 50%.
Typical electric Bilge pumps will not keep your boat from sinking if you have a broken seacock or a hole or some other disaster. They can give you time to figure out how to plug a hole or lessen the leakage and deal with the breach and get out a call for help. If you want serious dewatering capacity you will need another independent pump, likely engine driven.
*Make sure the wiring is heavy for the current drawn and the connections are well done and sealed well and out of the water.
If you want to nearly dry the bilge a centrifugal pmp, such as the ones discussed will not do that, unless the pump sits in a well/sump that is deep enough to allow the pump to run to the point the only water is in the well. Most of our boats have a fairly flat bilge which will require a different pump for that last bit down to a few cupsful or so.
Rule*seems to make the largest of the centrifugal type but there are others.
The Whale you mention is I believe a diaphram type.*They will pass junk that will stall a*centrifugal.* The centrifugals should actually have a screen around them to keep large junk at bay.
*I have a Whale, manual, double diaphram type for emergency use if the electrics fail.** The electrics *will give me time to set it up.* I have enough hoses with camlock fittings in place *to reach any bilge area and attach to an exhaust port in the hull side. I hope I never need it.
David Pascoe offers a good article to read also and goes into some detail:**** http://www.yachtsurvey.com/bilge_pumps.htm