Oh yeah!! This mount is definitely strong enough if you're going to get it really wet. I've had the jet ski roll over multiple times and be upside-down in the water for over 30 seconds each time with this mount on it and it has never even budged. I've hit waves that have pushed me back in the seat and this won't even move. If you're nervous about it falling off, I would recommend tethering it to something but it should hold if you make sure the area is clean where you're applying the mount.
In this video, I used the standard GoPro Suction Cup mount (shop.gopro.com/mounts/suction-cup/AUCMT-302.html). I also have a couple of the small extension pieces attached to the mount in order to give me that angle. Nothing special, just the GoPro brand mount. However, note that the website says, "Not recommended for surfing, snowboarding or other high-impact sports. Recommended for non-impact activities only. The Floaty Backdoor will not support the weight of the Suction Cup mount." If you are nervous about your GoPro being knocked off (I definitely am nervous about that), I recommend using a camera tether (shop.gopro.com/accessories/camera-tethers/ATBKT-005.html).
