I had one of those head straps but after the first use gave it away. I found it way to shaky to use, any slight head movement translated into footage which made me feel sea sick!!.. I used it above and bellow water.
Phil M, I know exactly what you are talking about in regards to rough water looking calm. Unfortunately one of the great things about the wide angle having such a field of view makes things look further away and smaller. Im not sure of the exact figure but our eyes see roughly the equivalent of 90mm lens on a full frame camera where the gopro is around 10-12mm.
To make the sea look as it does to the human eye you need to shoot with a more magnified lens, unfortunately with this the field of view drops and any shaking is amplified. If you carry a compact camera with a zoom onboard try filming a little from that (not at full zoom) to slot among the other footage from the gopro. The big wave action is normally filmed from much bigger boats, ships or even helicopters where the filming can be done from further away zoomed into a certain rough section.
Here from a recent trip you can see the water looks quite smooth with a little wave in the distance from a mounted gopro, my daughter is photographing the same wave with her compact.
This is how it looks from my daughters camera.
Most compacts have some sort of stabilising built into them, make sure you have it turned on if you give it a try. If someone else is able to drive the boat find a place where you can brace yourself and hold the camera with both hands and just try and get those few seconds of steady footage. A little shaking and movement can add to the effect so don't be to hasty on the cutting (editing).
Ive seen a few videos where people film clips of preparing the boat, the road trip to the boat and so on which all add to the interest. Most of videos I make are for my family in the UK to see so most drag on a little for non family to hold interest in.
Maybe we could include a few links.
This link is to a bit of footage I did showing the dangers of our local bar crossing even on a calm day, It was filmed with a compact.