True enough,but a character who is more active uses up more oxygen,and so drowns faster.Reducing SPD,in this instance,just means the character uses less END per turn,and takes longer to drown.(Which is why it isn't possible to reduce your character's SPD to 1 to take even longer to die.It also means that a SPD 1 character ,if someone were silly enough to buid one,would drown as fast as a SPD 2 character).