Pretty much all year round, although the north of Tenerife (Porto Del Cruz)
can get a bit wet in the winter.

I've been to Tenerife in December and had nice warm weather.

Another alternative would be Lanzarote.   There are many diving operators on
the island ranging from simple air diving right through to TriMix,with many
now at least offering Nitrox.  Many dive out of the old harbour in Puerto
del Carmen, even those based 15 minutes away in Costa Teguize.

I know Lanzarote well as we have a timeshare on the island.

Regards,

TonyH.

As others have suggested alternatives, I'll do as well (I haven't been
diving in Tenerife so I cannot say anything good or bad about it).

We enjoyed enormously staying and diving on El Hierro, which is a small
island SW from Tenerife. We stayed in La Restinga which is a small town
in the Southernmost part of the island. AFAIK it is the place on that
island for diving.

Off-season the place is pretty quiet, although the diving is still good.
On-season (apparently August) it is packed with mostly divers from
Canary Islands, and some Spaniards. We were there last October-November,
and there were other divers as well, but the atmosphere was really laid
back and relaxed since the place was really quiet.

It is easy to get there from Tenerife either by plane or by ferry. We
stayed at Arenas Blancas since it was the only place with a pool for the
kids. And I can highly recommend El Tambouril scuba shop for the diving
and helping with the other arrangements. Spanish is a must in most other
 scuba shops.

Hi Major!
You can dive on Tenerife all year around. I think the diving is wonderful on Tenerife. Imagine a boat dive and see a lots of rays, fantastic. The shore dives is also very good. Can recomend you to contact www.paradisedivers.co.uk     Friendly good dive centre.
Have a good holiday
Carina