I couldn't quite understand how the onboard camera's were placed, and clearly outboard of the hull and in such a manner to ensure stability ??
I used my cellphone for the one external shot starting at 1:52. It's a little awkward trying to film holding the cellphone while steering the boat at the same time. The onboard footage was all recorded using an old GoPro Hero3 in a waterproof case.
With a few different GoPro mounting accessories cobbled together into a gooseneck, the position and aim of the camera is easily adjusted. A little wooden clamp fastens the gooseneck anywhere along the boat's gunwale railing. The standard thread on the bolt matches any normal tripod mounting, so it screws right into the GoPro mount and tightens the clamp onto the boat's cap rail.
The video uses clips combined from multiple different outings, with the camera moved or merely re-aimed between sailings. One had the camera mounted looking forward from the stern rail. Then the camera was mounted midship hanging outboard, first looking rearward, then forward, or aimed straight across the deck.
In strong winds the boat will sometimes heel over far enough to dip the rail under, with any water then quickly flowing back out through the scuppers. But with the camera hanging outboard of the rail at midship, it will occasionally dunk the camera under. The camera is in a waterproof case. But with the camera dragging over the side, it does cause a lot more water than usual to splash across the deck. In the shots looking forward from the stern rail, when the camera is not hanging over the side, you can see there normally is hardly any water going onto the deck.
The weight of the camera hanging that far outboard doesn't seem to affect the boat at all. It's incredibly stable. You could hang a brick over the side and the boat would hardly notice.
I've even done a video with the camera suspended from the stern rail completely underwater. The only noticeable difference is that there's a lot more gurgling water noise, because the camera case is not particularly streamlined. See video:
https://youtu.be/tjqf6Kmk49A