Basic platforming is working satisfactorily. Today I spent working on implementing swimming and crawling.
Crawling is necessary because of the variable heights for rocks and sand; it will allow you to enter passages as low as 3 levels high. The tradeoff is that movement is much slower while crawling... which isn't a huge drawback really.
Swimming lets you move fairly freely in the water. Eventually the tradeoff will be that you can't live for long without coming up for air.
Both states actually change your character's dimensions a little. Since you're laying down, effectively, in both postures, your height goes down while your width increases. I could have implemented this as just a height change for crawling and a Mario-style vertical swimming pose, but it's a matter of detail.
That is the point where I think I'll be able to make a new demo video. It still wouldn't be what I'd call playable, because without equipment there will be many inescapable areas. That's by design -- real life cavers live or die by their equipment, and not every situation is escapable. In game terms, I'm planning on having a "call for help" option where you can call for rescue if you get trapped in a truly inescapable situation, but at a huge monetary cost. (Maybe even a cost that increases the more times it is used?)