1. Barracuda are NOT aggressive to people unless you do something dim-witted like chase them or poke at them BUT they are highly curious particularly where glittery objects are involved. I have snorkelled with barracuda a number of times and never been bothered. They will sometimes follow or parallel you but I think that is curiosity.
My husband had a barracuda who was blind in one eye nearly crash into him once (he was on the barracuda's blind side. I don't know who was more surprised by husband or the barracuda.
2. I have only seen a large shark ONCE when snorkeling. He looked at me, decided I wasn't a fish and took off. Once again my husband ( I think maybe the fish are after him) had a more exciting experience. He was out snorkeling by a barrier reef (lots of large fish) with our guide who was wearing a new bathing suit. Her suit was covered in what looked like tiny scales. Well guess what - about 6 very large sharks appeared to check out the "new type" of fish. She and my husband floated quietly while she was inspected and then returned slowly and carefully to the boat. She told me from now on that suit was ONLY going to be used for sun bathing purposes.
3. Jelly fish - saw a couple of tiny ones - never stung.