Paco:If you're a bit nervous about the drift diving, I would recommend that your first one be an afternoon dive (they're shallower) and ask about Paradize Reef (Paraiso in Sp.). This reef is teaming with fish and only 40' deep. I think that after that one you'll have the confidence to do others. The diving off Plaza las Glorias is generally in 15 ft. of water, but you can see some old Spanish cannons and some fish but it doesn't compare to the reefs.
Are you sure that those are cannons? I've seen some things off Caribe Blu that sorta looked like cannons, but they turned out to be pieces of old drainage pipe.