Here are some detailed instructions on building a DIY bilge pump...
DIY PVC Boat Blige Pump
As for a 'combo' pump that could act both as a bilge and a top off pump. In theory it may seem possible, but I think it would be difficult to pull off. Water and air behave very differently under pressure.
In one function you are trying to make vacuum, in the other trying to make presssure. So the valve system would need to be reversible. The size and shape of the valves that are optimized for air are quite different than the size and shape that would work best for water. .
Rafts are high volume, low pressure, most boats top off around 3 psi. Water pumps tend to be more low volume, moderate to high pressure. You couldn't move much water at 3 psi.
Most commercial air pumps have a big barrel, 4-5 inches in diameter. The big volume is needed because air compresses a lot before enough pressure is created to open the valves. Water on the other hand doesn't compress, a hand powered bilge with a 4-5 inch diamter would take some BIG muscles to operate.