It's funny cause I just had this discussion with the owner of my LFS
last night. Natural sea water will not instantly cycle a tank and in my opinion won't really help the cycle either. I think the main reason people use it is out of convenience as you don't have to mix it. If I lived at the coast I would likely use it but it does have to be sterilized and may contain contaminants which kinda eliminates the benefit. The show tanked is entertaining to watch and they build some amazing tanks but the way they fill them and add fish doesn't fly with me. The bacteria in a bottle helps but when you add huge amounts of fish to a new system and it "instantly cycles" because they used natural sea water or bacteria in a bottle is a load of bs. I'm sure those tanks suffer massive losses. When I started my tank I used cured live rock and added some seachem stability and my cycle was instant but I feel that was due to the rock and not the stability. Alot of people do swear by it and it likely does help but nothing beats a proper fishless cycle.