Re: I'm trying to set an alarm off when someone takes a bouy out of the water

The main problems here will be withstanding the environment, and
avoiding false alarms. You are right to be worried about corrosion -
The electrical part should be completely sealed from the water. Here 
are a few suggestions : 

1) Float-switch, where the float contains a magnet, and activates a sealed
reed switch. A possible problem with this solution is that dirt
and debris could block the movement of the float.
Manufacturers of ready-made floatswitches include GTI, Aseemtech and Crydom

2) Pressure switch, to detect the change in pressure at the bottom of
the buoy caused by its removal from the water. The switch could be
mounted on the bottom of the bouy, or a tube could be used with a switch
above the waterline or inside.  You may need to take care to avoid brief 
alarms
due to water movement, wind etc. One manufacturer of a suitably
sensitive mechanical pressure switch is Herga (P/N 6753-00)

3) Capacitive proximity detector. These are commonly used in industry to
sense the distance of an electrically conductive object, typically up to 
50mm
away. One of these mounted on or inside the bouy should easily detect the 
presence
or absence of the water. This would require a power supply.
Manufacturers include Honeywell, Carlo Gavazzi, Sick, Omron

4) Reflective optical detector (e.g. Honeywell LL-105100), which detects 
the change in
refractive index between water and air - dirt contamination could be a
problem though. Requires a power supply.

5) A thermistor, which has a small heating current flowing through it.
This detects the decreased thermal when removed from the water, causing
a change in resistance. This method is often used for 'low fuel' warning
lamps in cars. If you have power available, this could make a very
simple, cheap and robust solution.

Most of the above devices should be readily available off-the-shelf from
suppliers of industrial control equipment (e.g. www.farnell.com)