Been there last Oct and February.
Vis was maybe 40-50ft in February and maybe 20-30 in October.
Saw a few different fish, puffers, flute fish, some other various schools of fish that I don't recognize. I missed an eel and ray of some sort. Nothing too exotic.
Suppose humpback whales are a possibility with good vis around February although probably a bit further away from the Islands themselves.
A couple of small caverns to dive too... those were probably the most fun.
Access to Lovers Beach is highly restricted. Have to have permits and only stay for 30 min. Can't recall the exact number but only a hundred or so visitors allowed per day. Not worth it for anyone who can dive in my opinion.
Didn't see any divers going there but there are tour companies that get you to snorkel your way to Lovers beach if you go.
Depends what town you are staying in but Chica Loca out of Sayulita does the beach snorkel trip and seems to be well regarded. Not sure what else is available out of Punta de Mita.
Total of about 6 dives in the Marietas. It definitely could have been better but we had a good time.
I dove with Sayulita Entourage both times.