Losing Control Controls are another big problem, made worse by the game's reliance on pixel perfect jumps and close quarters combat.
Swimming is a total nightmare.
You use the jump key to move forwards while underwater, and the backwards and forwards keys angle your body up and down. This can be very disorientating, especially as the controls are different when you reach the surface. When your gun locks on to a target, strafing will cause you to circle around them instead of moving from side to side. Because targetting is erratic at best, in practice this just makes controlling your character even more difficult. Worst of all, whenever the game's frame rate drops off you get control lag.