This is a topic that can fill a website by itself and there is plenty of information available on websites to help choosing between waterproof cameras. Some of the important features to consider are: image quality and stabilization, battery life, ease of use, (remote, voice control and how much such features affect battery comsumption), GPS and other telemetry, quality and variety of camera mounts for boats and other planned uses
Devices/Methods that can be used for training in addition to GPS enabled devices and Video
these are “metronomes” for stroke rate. They can provide speed visually, with audio, and with vibration.
sense stroke rate and distance - possibly used in conjunction with a phone app.
From The Adventures of Rocky and Bullwinkle
Boris: “Is more genius then usual! Do I control it by linking to it with some plug in my cerebral cortex?”
Fearless Leader: “Uh yeah we were going to do that but… we had some budget cuts.”