The water in Silfra comes from the glaciers and has been filtered through the biggest filter of them all, the earth. 
We took the Golden snorkel tour conducted by Scuba Iceland, and it was the best organized tour we have ever been on. Finni, the owner of Scuba Iceland and our guide, is a native Icelander, did an excellent job of driving us around, providing us with information on history, culture, food and all the natural wonders of Iceland.
Our tour involved a stopover at Silfra, which is a water body between the American and European tectonic plates. We signed up for snorkeling, as we are not scuba certified. Finni did an excellent job of prepping us for the snorkeling session. My wife was a first time snorkeler and was hesitant after getting into the water , but Finni did an awesome job of handholding her and making sure that she completed the whole distance, after which she was glad she did it. So, first time snorkelers fear not with Finni around :).
After the snorkeling session, in some really cold water, Finni provided us with some hot chocolate and yummy sandwiches. Having been trained and spent several years in the US, Finni has excellent communication skills and make you feel at home,if you are from the US.