Went there with a couple of friends just before Christmas in 2015 and had a great time. The water was very cold (it was also -2 outside so it was to be expected), but they give you protective suit so you don't really feel it and you kind of forget about it once you're swimming in the water - a huge plus is that they give you a hot chocolate and cookie after the snorkelling!
This tour was incredible! Birgir, our guide, was funny, informative, so helpful and friendly. The freedive is perfect mix of snorkling and scuba diving. I highly recommend this tour - it's so beautiful in Silfra and I highly recommend using Freedive.is for your company tour!