Sharks are present in Komodo, but they are not generally considered a significant danger to tourists. The sharks most snorkelers encounter are reef species such as blacktip and whitetip reef sharks. These animals spend their time around reefs hunting fish and normally keep their distance from people. In practice, the current is a much more important safety issue in Komodo than sharks are.
That does not mean sharks should be treated casually. They are wild animals, and visitors should never attempt to touch, feed, chase, or block one for a photograph. If a shark passes nearby, the best response is simply to remain calm and let it continue on its path. Splashing frantically or swimming after it only creates an unnecessary situation.
If you are uncomfortable around sharks, tell your guide before entering the water. They can keep you close, explain what species are commonly seen at the site, and help you decide whether you want to enter at all. A good Komodo operator should never pressure someone into a wildlife encounter. Most people who are anxious beforehand end up surprised by how peaceful a reef-shark sighting actually feels.

