Some folks use a magnetic reed switch and relay combination for this. Engel offers them as part of their kits and (I believe) sells them separately.
For my OTW Type VII I used a simple slide switch that I placed in a 1-inch tube with a SubTech BHS seal on one end and the two terminals on the other. Think "mini dive module". The BHS seal has a brass rid through it that is connected to the slide switch...a simple push/pull turns the sub on/off.