Here's an inexpenSIEVE solution!
Bilge gunk can clog up a float switch or bilge sensor – and what you need is a way for the water to reach and activate it, but for the rubbish to stay away. The answer? It could be a cheap plastic sieve, attached to the floor and sitting over the sensor.
Visit Fox’s for all your chandlery needs – http://www.foxsonline.com/