If the need is to fill or empty the water with a pump, just fuse and mount a momentary contact switch adjacent the fishbox.
However if the idea is to use it as a livewell, do the same thing but use this instead of the momentary contact switch.
This livewell timer is fully adjustable. The "On" cycle will run for 30 seconds. The "Off" cycle will run from 5 seconds to 5.5 minutes. This unit has a continuous run option for filling the livewell. It also feature water tight solid state components. It has the power to run multiple pumps.
10 amp variable livewell aerator timer. 30 seconds "ON" cycle. Variable 5 seconds for 5-1/2 minutes "OFF" cycle. Continuous mode for filling the livewell. Capacity to run multiple pumps. Easy installation.
http://www.boatersworld.com/product/...ks-accessories
http://www.outdoorsuperstore.com/pro...sp?prod=306932