It is 75% likely that a water breathing potion will contain two doses, 25% probable that there will be four in the container. 

The elixir allows the character drinking it to breathe normally in liquids which contain oxygen suspended within them. 

This ability lasts for one full hour per dose of potion quaffed, with an additional 1-10 rounds (minutes) variable. 

Thus, a character who has consumed a water breathing potion could enter the depths of a river, lake, or even the ocean and not drown while the magical effects of the potion persisted.