I got a go pro hero 5 session. I have used the voice command extensively and found it to be very good up to about 1.5 - 2 metres but next to useless in windy conditions. I suspect I will find out this weekend how well it work on my kayak but I think the voice comman will be limited at sea. I'm not too fussed by this because I intend to buy a waterproof wrist remote. However, if you are planning on using only the voice command then you will find it a bit limiting in typical conditions found while sea kayaking, unless of course it's a flat and dead calm day.
The rest of the camera I cannot fault. The sound above and below water is extraordinary as is the picture quality and range of settings.