At the risk of giving a flip answer I think the secret is basically your level of confidence in the water.
You have to
1. Be confident that you aren't going to drown
2. Be confident that you can deal with the problem
of getting some water into your breathing tube
3. Be confident in the control of your body in the water
with the fins on.
Probably the best way to be able to do that is practice in a controlled environment if possible. If you have access to a swimming pool for example without a lot people in it or spend a fair amount of time in a calm area of the lagoon without a lot of wave action then you can get use to floating along using your fins to propel you. I don't really know how to explain the breathing portion properly. When I use a snorkel I take small "restricted" breaths always aware of the possibility that water might be coming down my tube. I can actually "hear" the water gurgling into the tube before it gets to my mouth so I know that its time to exhale sharply into the mouth piece and blow out the water. Sometimes the best of us will get a random wave that fills the snorkel so quickly that we get a mouthful of water and its necessary to stick our head out of the water and get some air. Once you get comfortable with just the process of using your fins and clearing your snorkel tube you can try actually going under the water a few feet which of course is going to fill the tube with water and then coming back up to a level where you can blow the water out of the tube.
You also need to be confident in your equipment. Some tubes are better than others (personally I don't use the valve types myself as I think they just cause trouble but some people like them). As for your mask fogging problem:
Is this a rental or a mask that you own? If you have a new mask it will have a clear film from the manufacture on the inside that you need to get off. Toothpaste and a soft toothbrush works well to get that off. Commercial anti fog products applied to the inside of the mask before you go help the mask from fogging (personally I have found that a small amount of Johnson's baby shampoo mixed with water and sprayed on the inside of the mask works as well as anything).
When you are 25 meters under the water you can't just take the mask off so its necessary to get mask flooding and clearing techniques mastered. This can also be applicable to snorkeling if you just puncture the seal enough to let a little water into the mask and let the water clear off the fog. If you then tilt your head slightly backwards and blow out through you nose at the bottom of the mask you can clear the water from the mask. (again something you want to practice is a pool or a shallow lagoon.
You didn't mention how good a swimmer you are. If you aren't naturally comfortable in the water that can be a big part of the problem. Perhaps you need to wear a flotation vest for awhile so you feel more confident in deeper water. This of course has the disadvantage of not allowing you to dive below the surface to look at something closer but it might boost your confidence for awhile. I know that the reality of all this is more difficult than just the theory but practice time is really the only way to get use to being in the water. Its like when i teach people to snow ski. They have to learn how to fall down and how to STOP. Once they get that part down they are then confident enough to learn how to ski. In your case you need to learn how to keep your mask from fogging and how to clear the water from the tube without choking.
As for the fear of sharks theres really nothing I can do to solve that problem other than suggest that as far I know there has never been a tourist attacked by a shark in the maldives. I think the sharks (being that they are hunted by the locals are so scared of people that they swim away from the sounds of boats and you are fortunate to actually see one up close).