https://www.vd.ch/themes/mobilite/na...n-des-bateaux/
Some might say the reason boats like this are not asked to register are because they are rowboats (assuming you haven't attached an engine). I think it's more likely they are treated under the category "engin de plage", meaning, apparently, "beach contraption", similar to an inflatable unicorn or flamingo.
To be absolutely sure, you could contact the SAN, although as with many Swiss government rules you may get a different answer depending on when you call and whom you speak with...
If the craft is classified as beach craft - IMO you won't be able to put an engine on it nor register it. Usually it's a binary thing - either you must register (and thus can use and engine) or you may not register (thus may not use an engine).
But definitely worth just checking with the authorities: if you were not planning to use an engine and they classify it as a beach graft - you're good to go, throw it in the water and have fun you'll just have to stay within 150m from the shore
Regarding beach contraptions, the plot thickens... according to the link at https://www.admin.ch/opc/fr/classifi...252/index.html , those only have one air chamber, so the Intex with three doesn't qualify. It still seems to be a rowboat though.
Also, in Geneva and Vaud the beach zone seems to be 300m. Not that the police boats are zipping by often to check.