Be careful, you are facing a danger of sinking (ahem)! Your requirements to stay sharp and afloat will no doubt increase, even in during such a short 'dip'. I can't honestly give you percentages, that entirely depends on the currents, water temperature and depth. The real danger, were you, let's say need to operate machinery, drive a power boat or need to rescue someone in difficulties on the High Seas, and administer artificial respiration, is that your body might be functioning energetically and efficiently, but your mind - which controls it - might have run out of steam, and be dead in the water! Please be careful... I hope you are okay, but I know only too well the risks.
If this is a one-off 'emergency' situation, I suggest you don't spare the methylphendiate (assuming you have plenty to hand?), or attempt to function with as little as possible - take as much as you need to keep safe, well and functional. However, once the crisis is over and the floods have receded, have a good, long break from it, to give yourself time to recover - both mind and body! You cannot of course keep it up forever... specially taking maths tests (shudder!). Best of luck...