Everyone seems to treat the bilge pump like a safety item which in reality is a mere convenience item to facilitate the removal of incidental water that collects in the bilge. I can see if someone wants to provide a backup to the bilge pump but would like for them to understand they are just providing a convenience back up. It is important to realize that the new backup pump must have its own separate dischage thru hull line. Avoid the use of check valves. In the case of a shallow bilge where the backup pump cannot be mounted mounted above the usual waterline You would basically just have two pumps working in tandem with a higher power consumption. The real world pumping capacity of these small electrical pumps are about 1/4 of their rated capacity. They are bench rated with no loads. When in use heat, load of lifting water and inneficiecies greatly reduces their capacity. These pumps cannot fight any significant water intrusion from a broken hose or the smallest hole in the hull. Another misconception is that the bigger the boat, the larger the pump should be when in reality is the smaller vessels that will fill up the faster. My recommendation, is that if the bilge is too shallow to properly install a back up pump it is not worth it to drill a new thru hull discharge port. The best safety practice is to frequently inspect all hoses, clamps and thru hull fixtures and valves. Promptly correct any rain leaks or others that may develop. Also skimp on the use of water when washing the boat. Water from dripping bathing suits, galley spills and plumbing leaks all end up in the bilge. Just for info, some boat owners install a 120V sump pump to be used at the dock when connected to shorepower. Now that can provide a bit of safety. Like others have recommended for safety you could also install a manual gusher pump that can be operated from the cockpit.