When you cast lumen while already having the effects of lumen, it resets the effect timer to 1 minute, but when you cast respiratus while already having water breathing, it doesn’t reset the effect timer, so if you cast it before your last water breathing runs out, you don’t get the time but the spell cooldown still happens. Respiratus should work the same way as lumen - and probably any other spells - and reset the timer to 1 minute