Just use batteries with low voltage and the worst thing that can happen is that your battery drowns in the water...
Having made soft and paper buttons, a student wanted to have a bathing suit with LED's. When jumping into the water the LED's should light up.
Besides from the problems with electronics and water, it looked simple: two wires into the water should make a switch. The battery and LED's should be in a plastic bag and sealed.
But it didn't work!
The water, apparently being too clean, had a resistance of several K...
Solving this problem is easy and the things you can do with it are fun. Make your own rain meter, make a jacket which lights up in rain, make your milk bottle interactive etc.
Step 1: What Do You Need?
wire (preferable not too thick)
transistor BC457C or something like that
a LED, around 3V
a resistor around 4K
Step 2: Schematics
The schematics is very simple, the main thing is to get the transistor and the LED right:
The principle of the transistor is simple:
it lets current through when the base (middle) is positive.
otherwise it does not conducts current so there is not action.
You can position the transistor by looking at its shape, a half round cylinder.
The next thing to position right is the LED:
long leg to the positive side of the battery, short to the negative side.
(A LED is a diode: it will light up only when the current is coming from the right side.)
Step 3: Soldering and Testing
Variations 1: bathing suit, rain coat, umbrella's.
Variations 2: use LED's which swicth on and of from themselves, the so called flashing LED's, use RGB LED's which change color.
Variations 3: use more LED's together: you can solder the second LED parallel to the first, it will only drown your battery twice as fast...
Variation 4: make a normal switch between the battery and the positive (or negative side), you can switch off when your bathing suit is in the closet...
This is basically a switch, so you can switch on (or of) with water what you can switch with a normal switch: arduino, relais, etc
Last warning: never never never never do this with high voltage!!!