Hello prblythe,
I work there today. Everything supergaijin has mentioned is still absolutely true, but I do want to mention dates: we only see mantas at the cleaning station from around mid December to mid March. The rest of the year -the whole year, in fact- you can see them every evening from the surface on the bridge connecting the two islands, but you can't get in the water with them.
If you are planning your trip during manta season, I would suggest doing your open water diver course in the first three days of your stay, and then you can dive on manta point almost every day. We have a videography service, so you can request a videographer to accompany you and film your dive with the mantas!
You are absolutely right in thinking tht this is an opportunity not to be missed. Of all the creatures I have seen, the mantas are still my reigning favorites!