A properly sized check valve should provide minimal back pressure and little reduction in volume. Aside from the obvious ability of keeping the water moving in one direction they will also protect centrifugal pumps from extraordinaryly high head pressure as well as back-spinning of the impeller at shut-down, the latter being a major cause of premature pump failure. Spring check valves are much more reliable and flexible in usage. I would not recommend a swing check valve. Your hand bilge pump is actually just a couple of checks valves.