The bubblehead charm is an alchemy spells, because it gives underwater the caster oxygen out of the water, thus changing the property of it permanently. Else it would have turned back into water and could have killed the user.
[ v ]Human magic
|Wand based||Alchemy · Charms · Duelling · Transfiguration · Wand lore|
|Wandless||Enchantments · Herbology · Necromancy · Potions · Rituals|
|Mixed||Divination · Healing · Magical theory · Mind magic|