I view the open water course, and certification as an Open Water Diver to be only a license to learn how to dive. You DO NOT learn how to dive in an OW course, be it a weekend or a semester. You learn techniques and skills that allow you to begin to learn how to dive, in a reasonably safe and comfortable manner. In a thread about open water training, probably 5 years ago (Looking Back - Did Your OW Teach You Enough?), I made the statement, 'I learned what I needed to learn, in order to begin to learn how to dive.' I still believe that to be true about OW training.
This is spot on! We have had a long development from (mostly) military divers and weeks-long training, to scuba training with broad appeal that is handled in baby steps. Once you get your ticket to learn, get Advanced Open Water certification to begin to perfect your techniques. As for Open Water, you will be safe if you stay within the limits of your training.
As for the one day class, that is the only way my shop teaches it now. Why? Because they couldn't sell longer class arrangements to the current age group of young interested dive candidates! No one signed up!
In our one-day marathon, we spend 10 hours total, with lots of breaks, and in class remediate questions that arose during e-Learning. But the student had to spend 3 days to a month leaning the theory from a text and video, or online course. Night classes, with multiple question and answer sessions would have been better, but no one signed up.
In the pool, we have a max of four students and one instructor and maybe a divemaster. LOTS of personal attention. Even with that, about 10% are suggested to arrange for some add'l one-on-one tutoring (at add'l cost) because they don't pick up the skills to be safe in the ocean. Otherwise, helping them along individually slows the whole class down.
Come ocean time, not everyone passes for the same reason.
Bottom line, one pool day with practice is certainly enough to be safe, but that assumes you are comfy in the water, don't panic easily and have paid attention during the book studies.
Then, start diving right away to practice, and get more advanced training soon after.
You get out what you put in.
As an old diver, I find that I can only offer what people want to buy. In my area, the way I'd like to do it just doesn't sell when people come to the shop.
But my private students get it over multiple days.