Not too necessary.
Odds are very low you will encounter a stinging organism this will help you with. Some few, of the tens of thousands swimming in ocean water in your vicinity of the world will get stung. Stinging organisms also don't discriminate where they sting, like places you won't likely apply SafeSea: palms of hands, bottoms of feet, hair and scalp, underarms, under the bathing suit and private parts...
Save yourself some $ and carry a small bottle of weak ammonia / water solution to apply to a sting if you are still worried about it - and actually hit the jackpot and get stung. Also, always shower with fresh water as soon as you can after getting out of the ocean to mitigate any "sea lice", if you are worried about that too.