Yes, there are sharks and landmines in Croatia, but it’s nothing you should worry much about on Vis. Just don’t go spearfishing with bloody fish hanging off your belt and stay on the beaten paths when you go walking (especially around deserted military installations) and you should be fine.
You can read about Damjan Pesek’s 2008 Vis shark bite ordeal here (chrome can translate it into broken English for you.) Sharks are called “sea dogs” in Croatian, so that’s why you’ll see the word “dog” in the article.
Fact: you’re more likely to get hit by lightning than to get bit by a shark.