I agree Gino.... a kayaker can be hit by a boater or anything on or under the water. I forgot. Giant rays and other lesser rays can also be repelled by the mentioned product.
Hmmm.... if i would be safety conscious, a good radar reflector or strobe lighting can be installed as a safety accident preventive measure. Radar visibility is possible as is visual Line-Of-Sight (LOS) strobe lighting in a mast.
A kayaker should know his/her limitations and bring safety devices. A fox whistle should be heard a mile away. Hand held compressed air horn is good too.
I warned some Hong Kong kayakers in another forum not to try it at night. There is the danger of the high speed ferries or even a fishing junk. Get into a high security restricted dock or shore area and you could get into trouble like being shot at or arrested as terrorists or saboteurs.
But don't underestimate the humble kayak. The Brits remember the cockleshell heroes and their commando raid at Bordeaux Harbor in World War II occupied France. Two-man kayaks armed with limpet mines and guts.
As the man said.... if you want peace of mind and security while on the water, it would be prudent to get a device then play russian roulette with a Great White Shark.