Hi Carol,
On your boat, you should have a bilge pump that will work both manually and automatically. Here is a 500 GPH Rule pump and separate float switch that I have on my 27' sailboat. It has been installed for 9 seasons and has never failed to operate properly. It should be wired to the switch panel so that it can be operated manually simply by flipping the switch. The float and the pump should also be wired directly to the battery so that the float switch will turn the pump on even when your battery switch is "OFF" and you have no power at the panel. You should have this assurance to cover any circumstance when you are away from your boat.
http://www.jamestowndistributors.com/userportal/product.do?part=117556&SHOPZILLA&CA_6C15C=613373747
Do not be tempted to purchase a pump that has the combination float switch integrated with the pump. My yard tried this on my boat during my first year of ownership when I replaced the old one and it failed miserably. They removed it and replaced it with the proper float switch such as you see in the link. The combo model failed within a month of installation. These may be fine for a powerboat that sits level almost all the time (I can get many years of service with this type of pump in my ski boat), but a sailboat that is heeling will end up burning out the float switch and/or pump or cause it to simply malfunction. From my experience, they simply don't work on a sailboat and will fail at the wrong time.
The separate float switch will not rise when heeling as long as it is oriented properly along the centerline of the bilge.
I know that this pump will work properly because I drain my ice box into the bilge (melting ice which I need to keep the beer cold). I typically empty the bilge manually with the panel switch because the float switch only comes on when the water is about 2 or 3 inches higher than the bilge discharge. It is designed this way to prevent the float switch from burning out the pump by running it dry. Every time I empty my bilge from the ice water, I wipe down the bottom and manually lift the float to verify that it is working properly. It has never failed.
I probably have not had any occasion within that past 5 years (since installing a PSS shaft seal) where the water has risen in the bilge high enough to rely upon the float switch (except when I have purposely filled the bilge) but I think it is absolutely necessary to have it in place.
Most sailboat owners also believe that there should be a separate back-up manual bilge pump that can be operated from the cockpit with a hand pump. I don't have one but intend to install one if I move the boat to a location where safety at sea is a more serious issue. For now, I simply have one of those plastic Beckson thirsty mates for my manual back-up pump.