I am creating an open source dive computer (www.diydivecomputer.com) and I am having a little bit of a problem trying to figure out how to power the device on and off. The problem is that the device needs to be completely sealed since it will be operating underwater and at high pressure (up to 200 psi) so there cannot be any holes in the case for a switch. I will be using hall effect sensors/magnets for the user interface buttons and would like to be able to power the device on and off using one or all of these buttons (i.e. press any button to power on). I would also like to be able to power off the device by holding down one of the buttons for several seconds (to reboot the device in case of a software crash). Lastly, I would like the device to automatically power on when the device is emersed in water. I will have connections leading to the outside of the case for charging so I could add another for water sensing (or somehow use the charging pins). Of course, the power consumption when the power is off needs to be zero or as close to it as possible since the device may be stored for months between uses.
I am a software engineer and electronics hobbyist so this is a bit outside of my capabilities. I know this is "question" with no simple answer but if anyone can point me in a direction, I would appreciate it! However, if you do find this an interesting problem I invite you to join the project! I could use the help!