The problem with magnetic mounts is that they work fine as long as you don't touch them, but if you bump them, they can pop right off. They're an all-or-nothing deal.
I have a Showdown ice fishing fish finder. It came with a soft nylon case. Because I often carry it in a trailer behind my snowmobile, where it can get bounced around a lot, I made a case for it out of a 30 cal. army surplus ammo can. When I'm transporting the fish finder, the screen is inside the ammo can with the battery. When I'm ready to use the fish finder, I take the screen unit out of the ammo can and stick it on top of the ammo can with a big strong magnet. Which works great until you bump the screen unit. Then it can fall right off the top of the ammo can. So I need to be careful when handling or transporting the fish finder with the screen unit stuck on the outside of the ammo can with the magnet.
The fish finder described in the video has a transducer base which attaches to a boat, and presumably to a kayak, with a pair of magnets. One magnet with the transducer base goes on the outside of the boat, and the other magnet goes on the inside of the boat. I predict that the transducer will periodically pop off the side of the boat. The transducer won't be lost, because it's attached to the fish finder with a cable, but it could be irritating replacing the transducer on the side of the boat periodically.