Silfra has some of the clearest water in the world. It's a crack between the Eurasian and North Atlantic tectonic plates. The water is melted glacier ice, filtered through lava rock which makes it extremely clear. Here you can go snorkeling or diving with a visibility of over 100m. It feels like flying.
The water is VERY cold, but you'll be safe in a dry suit provided by the tour company.
Plan to park at Þingvellir Parking P5 and walk to where all of the dive vans are parked.