Some advance exposure can be a good thing if it's provided by someone qualified (in no formal way) to be such a mentor. This is really a mentoring role we are talking about. In some ways a parent or friend taking a prospective diver into a pool is probably safer than stories of Dive Maters taking multiple customers on discover dives to significant depths.
Learning to dive is an adaptive process that is easier for some than for others. The 2 ingredients are repeated exposure and competent guidance. The mentor can easily provide ample exposure and a reasonable amount of guidance. You would hope that the instructor has a bigger bag of tricks but more often than not decent guidance and time to safely fumble with things is what it's all about.
Entering an OW class KNOWING that you can do this is very liberating and opens you up to absorb the presentation. This was my experience (and my wife's) by making local discover dives prior to our OW class. In a mentored situation I am not talking about time enough to "develop bad habits" It's more like getting past the first rush of adrenaline, anxiousness or other overriding emotion to let you absorb and learn good habits.
Many people do their best by pre-learning material. This may be academic or physical skills. They aren't trying to beat the system, they just want a little confidence and to get off on the right foot.
I have taken my share of courses but in many cases I had already done it competently beforehand and got little if anything at all from the sanctioned class. Self study and mentoring can go along ways. This sort of learning is very common certified divers. In the hands of a reasonably competent mentor lung over expansion is the biggest risk. In OW training this is really managed by a few mantras, not with a mystical sacrament performed by an ordained instructor. Keep breathing, don't hold your breath, blow and go yada yada. If were talking about a reasonably mature person that is mentally competent the risk is very low in shallow water. Yes there is a risk but that's life.
I'm seeing this in the scope of a domestic pool that may get to 10 feet and with ample shallow space to get comfortable with things.
This sort of prelude is not for everyone and I think for the most part the nervous Nellie will avoid it in favor of starting with an instructor. The ambitious self starter probably has the temperament to make it a positive experience.
Yes, a lot of this can be pre-learned
skin diving and I would encourage that foundation prior to getting near compressed air. That alone builds an underwater composure that makes the addition of scuba gear a mere convenience and an easy transition.
Pete
