Bag of Holding:
[…] Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate. (DMG pp. 153-154)
Cap of water breathing:
While wearing this cap underwater, you can speak its command word as an action to create a bubble of air around your head. It allows you to breathe normally underwater. This bubble stays with you until you speak the command word again, the cap is removed, or you are no longer underwater. (DMG p.157)
Cloak of the manta ray:
While wearing this cloak with its hood up, you can breathe underwater, and you have a swimming speed of 60 feet. Pulling the hood up or down requires an action. (DMG p.159)
Can a creature wear the cap or the cloak and hide inside the bag of holding for an unlimited amount of time (and not just 10 minutes)? I am pretty sure that the cap allows it, while the cloak description does not specify how the breathing process is performed
Both require you to be submersed in water to work. Now, if you were to fill the Bag of Holding with water first … I’d say the Manta Ray cloak still doesn’t work infinitely, because there’s only so much oxygen in the water you can breathe.
The Cap however creates a bubble of air around your head, and doesn’t insinuate in any way that the oxygen is taken from the surrounding water.