I'm not an electronics expert. I'm more a mechanical guy.
It seems to be really smart. If it works, it will solve a lot of problems.
Especially for those who sail much in seawater.
If the small version is not big enough. It´s also available in an extra strong version http://zepsus.com/magnetic-switch-15a/
But I probably settle for the small version.
The magnet is not a problem with credit cards, you haven't seen mythbusters?
Yes I think you're right, it will probably be fine in a submarine