As a kid, we used to sit on the bottom of the shallow end and look toward the deep end of the pool. We could see where the deeper water switched to dark blue. That's where Jaws lived.
Many years later I swam 2.4 miles in the ocean as part of an Ironman triathlon. At the pre-race meeting, a lot of people wanted to know about sharks. Could there be sharks during the swim? The announcer replied, "well yeah, this is the OCEAN, there could be sharks. But unless there is a school of 200 sharks cruising the course, we'll be OK." No sharks in my swim, but tons of jellyfish.
What scares you about triathlon? Where do those fears come from?
Here are some thoughts about fear in general to get you started.
Move beyond the fear, and try life. It's worth it.