Ken - let me start by clarifying something. I have ZERO problems with you or your training philosophy. In fact, I'd happily take a class from you, or recommend someone take a class from you. The reason I mention the overhead at Hudson is because I have been thinking a lot about different sites that people use for training lately, and there seems to be a theme of minimizing potential risks for the sake of convenience. (and PLEASE note - I'm not talking about you, nor am I trying to suggest that Hudson is a death trap)
Now to be clear, you stated:
The only course I use the 130' section for is AN/DP -- a course with a physiological overhead already, the pre-reqs to enter are over 75 dives and some technical training, and fully redundant gear.
I agree entirely here, and I feel this is a great use of Hudson! But at that level, I don't think you'd feel uncomfortable bringing students to Blue Grotto, either. Clearly this was not the focus of the discussion so far...
Now let's get back on the topic of using Blue Grotto for the deep dive in Advanced, or the cavern for OW training.
For open water training, I feel like this is a slam dunk. Blue grotto is fine for some open water work, so long as you actually stay in the open water - and that available space is rather small. Overhead is clearly present, even down to the air bell, and open water students don't belong there. As clearly posted in bold black (and highlighted) above by Jim Wyatt, PADI standards forbid overhead for open water dives.
Advanced is POTENTIALLY a different story. Technically, conducting a cavern adventure dive during the AOW class is allowed IF the instructor is a Cavern instructor, and the student is a minimum of 18 years old:
Section 2 of the PADI AOW curriculum under Adventure Dives Conducted Using
PADI Specialty Diver Course Materials says:
"When using specialty diver materials for Adventure Dives
not supported by PADI Advanced Open Water Diver
materials, meet these requirements:
1. You must be certified as an instructor for the specialty.
2. Student divers must meet all prerequisites and other
requirements for the specialty.
Exception:
Student divers do not have to be Advanced Open Water
Divers for the Cavern, Ice or Self-Reliant Adventure Dives."
A chart then follows, which states that for Cavern adventure dives to be conducted, the instructor must be a certified as a Cavern instructor, and the student must be a minimum of 18 years old.
From section 3 of the AOW curriculum:
"Do not conduct Adventure Dives in any situation where
direct access to the surface is not possible.
Exceptions:
Ice and Cavern Adventure Dives."
So yay! With the proper intructor, newly minted OW divers looking for an AOW card can go on ice dives and cavern dives! Yeah...I just threw up in my mouth a little, too.
Under the specific section regarding Cavern Diver adventure dives, though, there is a potential sticking point:
"4. If entering a cavern, limit the dive to within the light
zone and within 40 metres/130 feet from the surface,
vertical and horizontal distance included."
How this is interpreted makes all the difference if you can use the bottom of blue grotto for AOW Cavern adventure dives or not. My understanding of this combined distance is that it is depth plus horizontal penetration, not necessarily the distance of the straight line you would swim to reach the surface. If you really can count BG as a straight line of 130 feet to the surface, albeit a roughly 45 degree line, you may be OK...unless you are at the back of the cavern, in which case you are closer to 140 feet from the surface (assuming about 100 foot penetration to get to 100 foot depth). If not, about 60 foot penetration and 60 foot depth would get you to about peace rock and then you would have to peace out, even for PADI cavern.
All that considered, I am willing to bet that the vast majority of instructors using BG for deep dives aren't cavern instructors...that, and the thought of taking a rather newly minted OW diver looking for an AOW rating on a cavern or ice dive makes me cringe.
Let's go back to Hudson for a sec (map above). There's a down line and platform at 100' right in the center of the sink. Instructors can easily use that platform to accomplish the deep dive in an Advanced class without ever getting near an overhead, but may prefer to use Blue Grotto.
Yup! You can clearly do a deep dive at Hudson and stay within standards. I also feel that the less than crystal clear water at Hudson makes it easier to keep folks honest for the underwater nav stuff. If it weren't for the smell, I'd really like Hudson for AOW training. Close, cheep, deep - a pretty nice combo, imho!
How is this even similar?
Here's the crux of the issue. Convenience leads people to make decisions about training locations, even when some minimizing of risks at those locations is needed to make that decision OK. Standards for PADI clearly state that overhead diving is NOT OK for open water dives. It also appears that unless you are a cavern or ice instructor, overhead is NOT OK for AOW adventure dives. How much overhead is OK, and in what kind of conditions, is where the slippery slope starts.
If one did a tour of the lines on the bottom of Hudson on an AOW deep class, even on the shallower side of the sink, you do get into SOME overhead. By the standards, unless you are a cavern or ice diving instructor, and are conducting that adventure dive, you are now violating standards. Sure, the overhead is minimal, especially on the shallower side where the AOW folks could potentially go, but it's still there. Is this the same as going under a duck under, or swimming under the boat? Hmmm....maybe, especially when considering the slope of the wall there leads you to the surface. Then again, the explanation referenced above mentions good visibility in regards to where the surface is, and that doesn't exist at Hudson. Of course, it gets black as night down there, which is why they require 2 lights per diver to cruise the bottom. In a newly minted OW going for AOW diver who is doing a cruise around the bottom at Hudson, is it POSSIBLE that a light could go out, the student could start to panic and ascend, hit the sloped ceiling above them and freak out worse, with a bad outcome? Hmmmm...maybe again. You can always argue, especially with a good pre dive briefing and appropriate supervision that this is highly unlikely, but not likely doesn't eliminate risk.
Going back to BG: Is it LIKELY that a certified OW diver at peace rock would panic and not be able to find their way to the surface at blue grotto? It doesn't seem like it. But since when do panicked divers do what we think would be common sense? But I think that it is easy to convince ourselves that something is safer than it really is. I'm not convinced that larger overhead with clear visibility is any better or worse than minimal overhead but cave equivalent darkness, plus minus stirred silt from the bottom of Hudson from a panic stricken diver.
Extending this out even further, to sites like Devil's Den, Paradise Springs, and even Eagles Nest ballroom - it's easy to say that the dive plan is to stay in the area where surface light can be seen, and that it is safe. When considering Eagles Nest, you could even argue that someone could keep constant contact with the line to the mound, and always have a direct route of ascent to the surface, without rock technically being DIRECTLY above them, but I know we agree that EN is not appropriate for an AOW deep dive, even with a cavern instructor!
I guess my point of all of this, and certainly my reason for bringing up the overhead in Hudson, is that it's easy to forget that dangers, when perceived as minimal, are actually dangers...and that allows people to justify making minor deviations from the standards...and that increases risk for all involved.
...the horizontal penetration doesn't extend beyond 10'...
I'd be willing to bet you a frosty beer that the overhead is several times larger than you think it is. Next time you're there, run a knotted line from the 100' platform down line to the wall at about 45 feet. Then repeat that out to the far corner at the bottom. Just do me a favor - if I lose this bet, make sure you get a good shower in before I have to settle up with you.