Loose bilge pump in a Chrysler 26

[simon A]

The bilge pump of the sailboat I bought this year is loose. It holds only by the electric wire and water hose. There is water in the bilge, but turning the pump on result in a fine working pump that sends only air out. The pump came loose before I bought it and needs to be fastened somewhere below the water line in the bilge.
It is really hard to access: way forward below the floor (maybe 4 inches high). How can I fasten it to where it can do a proper job?
Simon