Do some research before you go! Not just on what to do but more about your surroundings.
We went to Jekyll Island, GA and while we were swimming I kept feeling something bump against my leg. We were there for a week and it would happen a few times a day every day we were at the beach. I thought it was my sister and never thought anything of it. The water there isn’t clear like in southern FL and the Caribbean. The second to last night we were there we went on a dolphin boat tour of the island and it turns out the beach we were swimming at all week was the number one breeding ground for sharks and it was breeding season. The bumping up against my leg that felt like sandpaper were really sharks. Had I known this before I would have been way more careful in the water! (side note- unlike in the movies you never see sharks coming, they don’t expose their fin above the water. If you see a fin it’s a dolphin. A tour guide told me that in Marco Island, FL.)
Another example is if you are hiking, check out plants to steer clear of and make sure you remember what Poison Ivy, Oak and Sumac look like. Look at bugs such as poisonous spiders so if you get bit by one you know to immediately go get help.
Always remember to do your homework. Better be safe than sorry!