Don't sweat the sharks and barracudas, they really don't want such a big meal that they would have to work at eating
If they were really a danger, a lot of us in this forum would have been dinner long ago
As for rip tides/strong currents, if you get caught one do not , I repeat, do not fight it. You will tire out long before the current does. The trick for getting out of such currents is to swim at a right angle to the current till you get out, even if that means swimming away from the shore. Odds of finding such currents in good snorkeling areas are rare though. Rips are normally a feature of heavy surf, not exactly a good spot for snorkeling
Avoiding heavy surf will also help prevent you from getting smashed on the reef. Dustin is also right about wearing a wetsuit or a skin, it will protect you from stings if that worries you. A wetsuit is buoyant, so if your a weak swimmer that might be a good idea also