Spain diving has big variety of diving sites. You can find from cold water diving to all year round warm water diving in the Canary Islands (off the west coast of Africa) and ofcourse, Mediterranean diving.
Average air temperature during the year: 5°C (41°F) - 31°C (87°F)
Average Water surface temperature: From 10°C (50°F) to 24°C (75°F). In summer you'll need a 5 mm wetsuit, in winter a 7 mm wetsuit or drysuit.
Coldest time: January and February
Warmest time: July and August
Possible to dive all year round
Best time to dive: Summers are hot, Spring and Autumn mild. Best time for diving is May to September, although the Canaries offer year-round diving.
Visibility average: 12 to 30 meters
Spain diving offers diving sites for all levels. Rocky reefs and wrecks; drop-offs and walls; caves and caverns.
Diving is available all round Spain's coastal areas. Famous is the protected Medas Islands off the Costa Brava, the Costa Blanca, Costa del Sol, Majorca and Minorca islands and the Canaries islands. Gibraltar has a number of dive sites including a nice reef and vertical walls in the Straits, where large pelagic species can be seen.
Around some sites off the Atlantic islands and in the Straits of Gibraltar strong currents can be encontered!
Diving in the marine reserve of Medes islands has some of the best diving in Spain. These islands are reached by boat from L'Estartit, Costa Brava.
Undersea flora includes gorgonia; sponges; false coral and red coral in some areas, fishes are a mix of Mediterranean and Atlantic. Fish commonly encountered: octopus; rays; barracuda; scorpionfish; groupers; amberjack and morays and congers. Large Atlantic pelagics in the Canaries islands.
I just wrote a few highligts but you can find more in the regional pages or in the