Airy Water

The airy water spell turns normal liquid such as water or water based infusions or solutions to a less dense, breathable substance. 

Thus, if the magic-user were desirous of entering an underwater place, he or she would step into the water, cast the spell and sink downwards in a globe of bubbling water which he or she and any companions in the spell's area of effect could move freely in and breathe just as if it were air rather than water. The globe will move with the spell caster. 

Note that water breathing creatures will avoid a sphere (or hemisphere) of airy water, although intelligent ones can enter it if they are able to move by means other than swimming, but no water-breathers will be able to breathe in an area affected by this spell. 

There is only one word which needs to be spoken to actuate the magic, and the material component of the spell is a small handful of alkaline or bromine salts.