Light
Additional light is necessary to see anything in deep water.
Halogen lamps
Halogen lamp and Luxeon LED: Euro coin for scale.
Cheap and bright. Easily available in any Home&Garden shop. 50W/12V halogen lamp costs about 1€. The only problem is power consumption. For 50W/12V lamp you must supply 4A. The way you can limit power consumption is to use PWM, this gives you the possibility to limit the brightness and power according to your current needs. Be warned that lamps are very hot and can damage your plastic parts.
Note that lamps like this shown on the picture are sealed. You must only isolate wire connection and lamps are ready to put into water.
LED
Luxeon III emiter
Nowdays, there are super bright LEDs available on the market. They consume much less energy than halogen lamps and have low heat emission.
Luxeon is one of the popular products. They offer 1W, 3W (Luxeon III) and 5W (Luxeon V) LEDs. For example 3W Luxeon III delivers 80lm@1000mA.
Luxeon Emmiter is only diode, Luxeon Star is diode and star like heat-sink platform.
Cheaper alternative for Luxeon is Edixeon. It provides even more light than Luxeon LEDs, for example 3W LED gives 120lm@1000mA.
For comparison, standard 100W bulb gives 1300lm.
There are other superbright LED solution, e.g. OSRAM LED solutions.
All those LEDs need heater.
It can be helpful to look at LED Resistor Calculator (Luxeon website) to compute value of resistor to be used with LEDs.
