Agree with all above, but just to clarify... You should NEVER need to turn the switch to ON for your bilge pump. Assuming the pump is wired correctly, it will turn on automatically when there is enough water to pump out. It does this by an internal float switch. Float comes up, pump starts. Float goes down, pump turns off. The switch in your dash is really only there for if your float fails. If you're concerned about its operation, its usually easy to check. Most bilge pumps have a float test dial. Basically you turn it (which lifts the float) and the pump starts. If you do that and it starts, you're good. If it doesn't have an external test dial, most just snap off their base to reveal the float. Manually push up on the float, if it turns on, you're good. If not, check wiring or replace.
However, again I do agree with guys above. You almost certainly have a leak. I'd start by checking with ballast connections and prop shaft seal. Good luck!