The weather in Iceland is totally unpredictable. Even when the sun is shining, there’s a fair chance that it will turn to a cold and wet weather within a matter of hours. In summer the temperature can be 15°C, in the winter it can be -7°C. From September to May there can be snow instead of rain because of the temperature. The south and west coasts are usually the wettest part of the country. Mid-June to August is high season. Because of the snow most highland tours don’t operate until July. Most tourist facilities outside Reykjavik are only open from late May to early September.
If you want to dive in Iceland you can do that all year long. The best time depends on what you want out of your dives. In the winter you can dive in the snow and get out of the water in the snow. In de summer the snow is melted and the country turns into green. Tourist season is during summer, which is between May and October. In Silfra the water temperature stays the same all year round. Ocean diving in Iceland is a bit different. The visibility is better in the winter because of less plankton in the water. That also means that there is less marine life to see. In the summer the visibility is not as good because the plankton is back, but there’s more marine life to see.
The water temperature range from 3°C to 8°C, so you will need a dry suit. At some places the water temperature can be higher because of the volcanos or hot springs in the area.