I know this can be done, I just can't put my finger on exactly how to wire it up. I have an underwater housing that I seal up indoors and take out on a boat. The device inside needs to be turned on before putting the housing in the water and turned off when we retrieve the housing maintaining the seal.
I have seen other devices that use a two reed switches and a magnet on the outside of the housing to accomplish this, but I'm not sure of the mechanism that keeps the device powered on after the "On" reed switch is triggered. I am currently looking at a dual coil latching relay that I think will accomplish what I'm trying to do. I've tried to illustrate this with MS Paint in red. I think each reed switch needs to be in their own circuit with the battery (its just poorly drawn here).

My questions are:

1. Am I on the right track with the idea about the dual coil latching relay?
2. Is there an easier way to accomplish this goal without a relay?
3. Do you have a suggestion on where to acquire the relay if that's the right way to go? DigiKey?

 

Welcome to AAC!

Yes, a dual coil latching relay should work.
Here is one of many examples.
You need to select your set/reset operating current and voltage, and the load current and voltage.
Plus there is the relay form factor and mounting styles to choose from.

https://www.digikey.com/en/products/detail/panasonic-electric-works/SP2-L2-DC12V/275310

 

You can also do the same thing with a couple of MOSFET Transistors .........
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Another possible solution is a suitable SCR. One reed switch could switch it on by pulsing power to the gate. The other reed switch could switch it off shorting anode to cathode. If the current was too much for the off read switch a transistor or mosfet could be turned on be the reed switch.

Les.

 

How about automatic switching? Capacitive sensors/switches known to 'switch on' when in presence of water on surface.
Drop box in the water and it's turns ON. Pull it out and dry it with rug and it's OFF

 

Depending on the load current and the physical arrangement, a simpler non-electronic scheme is a single higher current rated reed switch glued to te inside of a non-ferrous part of the housing, and a scheme to keep the magnet firmly in place on the outside opposite the reed switch. About as simple as it gets.
I also have a beacon strobe for a boat the normally hangs fresnel lens down, but will float lens up. It uses a mercury switch to activate the strobe while it is floating. In a really rough sea the strobe may occasionally switch off for a few moments.

And now the mandatory safety warning: Mercury is toxic to humans, therefore do not eat the mercury switch.

 

Another option is a mechanically latching relay, or one of those used to control lighting with multiple push buttons. Pulse one coil and the contacts close, pulse the other coil and they open. The pulses are momentary, usually 24 volts AC, so 12 volts DC should work well. The contacts are rated to handle up to 13 amps, usually, at 120 volts AC. They should serve quite well for 12 volts DC at less current.