My idea is to create a system that samples a accelerometer and can detect when a person (IE: child) enters a pool. The device would then connect to the home WiFi and ping the associated user.

The form factor would be the same size and shape as a tennis ball. It would float on the surface balanced such that a small solar panel would always face upwards.

Power budgeting will need to be completed first. I plan on storing power in a small super capacitor. I believe that a low power FPGA can sample the accelerometer and when needed trigger an interrupt on a low power MSP430 to send data over WiFi.