Secret Beach, pure paradise

No need to fear fish. Sharks will normally swim away from humans, and we're not in their normal diet. Shark bites are accidents, not attempts to take a life.

As for advice regarding diving, you should do everything you can to get certified before going under. Most dive shops and dive masters/dive boats won't allow you to dive on SCUBA unless you're certified. While diving is a relatively safe sport, it takes a fair amount of knowledge to avoid getting injured or even dying. Most fatalities and accidents involve non-certified people diving and getting decompression sickness (the bends). This occurs from gasses precipitating out of solution in the blood from either rising to fast from depth, or staying too deep for too long. Dive tables and computers are used to prevent the occurance. If you're not familiar with the prevention of decompression sickness, you really should not dive. Getting certified includes exhaustive classroom time regarding these hazards.

I'd strongly advise you to do it right and get certified before you go.