Miners have ingenious ways of getting rid of water. I've seen a few mines with culverting down the side of the passage, you can in some cases hear the water gurgling down the culverting.
We uncovered a culvert in JH West, about 30m from the entrance shaft when we were beavering away in there. It drained the passage in front by about a foot and a half to floor level.
If the culverting fails either by collapses in the roof or simple accidental blocking, it will fill up with water.
we've also seen drains cut into the side of the passages and entrances to direct the water into a more useful (and less splashy place). A key example of this is the entrance to Son of Longcliffe, where the miners have cut a drain in the rock around the drop and then allowed it to enter the drop where they wanted, thus they have reduced the chance of Random drips dropping on you.
There will be more examples of little drains cut into the rock in mines that we pass quite a lot, but the silent job they do will either be redundant or unnoticed to the passing visitor.
As for Mechano, I don't know, it's still on my reserve list. I suspect there could be lower levels as we've seen what looks like a winding station in Hillocks.
Crimbo Swallet is exactly that, a swallet, where the water disappears down a natural section, the miners would have doubtlessly exploited it to their own end. But whether they were ever down there prospecting is debatable.
much like the miners in Oxlow probably visiting new Oxlow once to check for lead, but when it would have been seen to be lead free they left.