You want a sensor in every compartment. If in doubt install two.
You can have ONE bilge alarm and connect all the sensors in parallel to it, so any one alarm will set it off.
It would be best if the alarm had it's own battery, or maybe run it from the starting battery; so if you have a small leak and the pumps run the house battery down, the alarm will still go off.
Ordinary float switches work for me; mounted above the pump floats so they'll only go off if the pump can't handle it.
I use a 12v auto siren for the alarm, it'll wake the neighbors. I also installed a "silence" switch with a light, too.
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw