The issue with depth is 1) you can't do a direct ascent to the surface past...18m or so?...with a high level of safety, for a large number of users, and from 30m or so (the recreational limit for most programs) it's even harder, plus that's the upper bound where nitrogen narcosis starts to set in. I've done down to 40m on air which is the next limit and the point where a lot of people have more serious nitrogen narcosis. Beyond that you really should be doing decompression diving and start to be looking at different gas mixes which makes it all a lot more complex. At depths beyond >50m oxygen toxicity on air would be bad too. The current normal limit is ~56m on air, previously people have done 60-65m pretty consistently on air, and the record is probably 155m (but that is insane and almost certainly caused harm, plus had high risk).
Ascent from depth without having cavities open to equalize pressure will be a really bad deal even from 18m (and really from comparatively shallow depths like 3-6m), but that really just means breathing out at slow rate during ascent. For that it'd be explode vs. implode though :)
