Getting an initial score of less than 50% is not a random event or an act of God. It illustrates that the child either a) can't do the work or b) didn't do the work. In the first case, the student needs remedial help and probably shouldn't be in that class. In the second case, the student is willfully not learning. That's the initial problem that needs to be addressed.
The analogy about being underwater is inadequate. Again, it presupposes that you suddenly find yourself underwater and asks the question: would you rather be 6' underwater or 600'? Well, obviously if I'm mysteriously appearing underwater I would rather be 6' underwater.
But in this case I'm actually starting on a dock, not underwater. I'm also carrying a big-ass pack on my back that weighs 100 pounds, and there's a tall boat about to set sail. I have several choices:
1. Fall into the water and sink to 600'. Requires minimal effort.
2. Take off the pack, then fall into the water and sink to 6'. Requires a bit of effort to take off the pack.
3. Take off the pack, then walk up a set of stairs to the deck of the boat. Requires more effort...but not THAT much effort.
All of this is explained to me in advance, so I know what the repurcussions are. I have to make a choice to move forward. If I make a bad choice, that decision making process is what needs to be addressed. Kids should know that if they don't study and don't do any work that they're going to get a 0 (or a 20%, or a 40%, or whatever). The process doesn't start AFTER they've received that grade...it starts BEFORE they've received that grade when they're deciding whether or not to attend class, study, do homework and take the tests.