Unending Breath is a core warlock ability learned at level 24. It is a spell that warlocks can cast on themselves and their allies to help them breathe underwater for 10 minutes. For those with the affliction specialization, it allows water walking instead via [Soulburn].
While walking on water, the spell does not break on damage. It can be used while mounted as well.
[Soulburn] ( affliction spec ability - water walking)