Mine wouldn't drain at all when I got her. If you're set up similarly, it is about the easiest fix in the world. My shower has a straight pipe that dumps into a fiberglassed compartment just forward of, and to the centerline from the shower. I removed the strainer, poked a coat hanger into the hole, and "whoosh", all the water drained into the compartment.
Hint: Test the bilge (sump) pump and float switch that is in the compartment BEFORE trying the above solution. On mine, it didn't work, and replacing it would have been far less distasteful without several gallons of gray water sharing the hole with the pump and float switch.