The drive from bcn to diving center (Unisub?) should be about an hour, from there, boat navigation is pretty short as Medas Islands are near de coast.
If your plan is a north-south drive through the mediterranean coast, these are the dive spots I won't miss (northern first):
- Columbretes Islands. You can do this dive with Barracuda Dive center (
Barracuda Buceo ::::) in Alcossebre. You must book this dive in advance as a special permision is needed to dive the marine reserve of Columbretes Islands. The most usual is Columbretes is a full day trip with several dives because navigation is quite long.
- Mediana Island. A very small island near Altea. You can do this dive with the guys from Celacanto dive center in Altea.
- Benidorm Island. I always do this dive from a dive center named Ali-Sub, in Villajoyosa. Several spots in the Island, best of them is the one named "La Llosa".
- Cabo de Palos. All dives in the Cabo de Palos reserve are fantastic, if you like wrecks, don't miss the one named "El Naranjito". You can find at least 8 dive centers in the marina of Cabo de Palos town.
I haven´t dived southern than Cabo de Palos, but I've heard good things about "La Azohia", "Mazarron", "Carboneras, "La Herradura"...
Tabarca Island is not worth a dive, I have dived there several times and has always resulted in a total disappointment.