I use loggers by Eagle tree. These record data to memory like voltage, current, motor RPM, temperature. Can also record other things more valuable to flyers like GPS, altitude, airspeed.
It's a handy device as you can see very accurately how the motor is performing with a certain prop combination in a dynamic setting, as opposed to running it up in a bucket of water, which only gives a rough idea.
Eagletree also offer devices that can work transmit data, but these are no good in subs as the frequency is too high. However to be honest, I don't think I would use telemetry much, I prefer to analyse the information on a laptop, and the view the data plotted on a graph.
The water switch sounds like a good idea, haven't seen anything else quite like that on the market. What sort of price could you build it for though? I'd say it would need to be sold for about £20 if you want them to sell.
I think the bottom line with this sort of thing is, do you want to make items to sell and make money from, or do you want to develop products as an intellectual pursuit and maybe sell a few as a paying hobby?
I see a lot of people saying 'build this or that', but when it comes to fruition and it's time to cough up, they evaporate like the morning dew.