Scuba diving in Silfra (Thingvellir National Park) is a stunning dry suit diving experience that will literally take you down into an underwater crack in Earth - the one that divides the North American and Eurasian tectonic plates. The Scuba driving tour includes a prior introduction from a knowledgeable guide. Inexperienced divers are asked to take a one-day diving course prior to the Silfra diving tour, to ensure their safety and optimal experience.
If you are not eager to waste two days on-site (which is, by the way, worth it), you can always choose to do snorkeling instead of scuba diving.