There's at least 2 different problems being addressed. One is a catastrophic hole, the other is maintenance
Catastrophic, you want the biggest pump you can fit. One surveyor
points out it takes a lot less water to sink a small boat
than a big one. As others have pointed out if you're not at the boat or have an alarm
that someone will do something about, a big pump won't do any good. A diaphragm
pump isn't big enough for this use.
For small amounts of water a maintenance
pump can be small. A diaphragm pump will pump air and water, potentially making for a dryer bilge and it has 2 check valves in it, less worries about backflow. If you go with centrifugal you want a small pump as all the water in the hose will drain back into the bilge, so smaller pump, smaller hose, less water remaining in bilge when pump turns off.
I also like the 2 pump idea, I have a diaphragm pump for my deck leaks
and a big centrifugal mounted higher for sinking.
You want an hour meter on the pump to let you know if leaks are getting worse while you're away.
One argument against mounting the outlet as high as possible is pressure head
is determined by the difference in height from the inlet to outlet. A centrifugal pump's output is affected by pressure head
. It is not determined by the highest point of the hose.
How big is big