To add to the myriad of options that have been thrown into the ring here, the 4iiii company intends to have their new Viiiiva Mini available on December 10.
For those not familiar with 4iiii, in the running and biking world, last year they brought out an interesting product, Viiiiva that on one level is a plain old chest strap that is a Heart Rate Monitor sender in the common industry standard ANT+ radio wave protocol. But in addition to that, it also listens for ANT+ signals sent by footpods, bike cadence and speed meters, power meters, then re-transmits these same signals (as well as its own internally sourced HR signal) in the new Bluetooth Low Energy format, so the data can be viewed in real time by those with apps on their iPhones and other Bluetooth LE devices.
Well, the Viiiiva Mini does all the latterly described ANT+ to BLE functions, but it's worn on the wrist, not on the chest, and derives the HR from monitoring the pulse by optical means. It will display the HR in real time, and will send it by ANT+ and BLE signal, but I would imagine the signal would not be received over any significant distance underwater, unless your receiving device is worn on the same wrist, next to it. (However, I am not aware of any swimming device set up to receive HR data in ANT+ even if it was worn on the wrist adjacent to the HR signal sender).