Most public beaches will post a jelly fish warning if there is an issue . Most snorkel or scuba operators will be aware of them as well.
Sharks generally do not consider humans for food. Most attacks happen at dawn or dusk when sharks are feeding near the surf line and mistake your toes for dinner. Surfers have some problems as they look like yummy seals to the shark from underneath.
Most tour operators are not going to take you somewhere where either shark attack or baracuda attack is a problem or issue. If you see either in the water, be aware of their presence , do not menace or swim towards them and you should be fine.
If you see either in the water, put yourself so that really annoying guy on the bus trip that morning is between you and the shark or baracuda. Who cares if he gets swallowed up? Seriously, you would read about it in the papers or see it on sixty minutes if any of this was a real issue.