Ecologic divers does night dives at HolChan and outside the reef.
they do not do snorkeling at night.
if you want to snorkel and dive this is best done during the day and at hol chan, yes they will take out both divers and snorklers.
i belive most dive center also do the same
you can check out their web site to see the various dives. www.ecologicdivers.com