I get people asking me if it's OK to book ahead from time to time... the answer to that is that I'd prefer it, I suspect that's the same for most if not all dive operators. I've had people book ahead as far as 14-15 months or so, but those are rare. Right now I've got numerous charters partially booked through September, September itself is starting to look fairly busy, but most people seem to be booking a little closer to actual dates than in years past. It's usually fairly easy to slide into an open spot on relatively short notice if there are only one or two of you in most cases, but with larger groups it can get dicey if you wait too long.
All in all it's somewhat of a guessing game. I usually recommend to people that they try to book when they feel comfortable that they know they definitely are planning on diving... during busier seasons it sometimes takes several phone calls to find space with someone if you wait too late.
I thought I'd mention this because I had a surprisingly busy May and had to turn down last minute bookings (by last minute, I mean last few days or longer) several times due to a full or mostly full boat. I'm not discouraging last minute calls, definitely give me a call (808-937-1175) if you're looking at diving last minute, but I'm definitely encouraging a little lead time if you plan on diving for sure.
Here's another shot of a turtle getting cleaned by surgeonfish/tangs at a cleaning station.