Deeproot Waters is a uncommon Enchantment card from Ixalan (XLN), collector number 51. Current market price: $0.38 (foil $3.14), TCGplayer market prices via Scryfall.
Whenever you cast a Merfolk spell, create a 1/1 blue Merfolk creature token with hexproof. (A creature with hexproof can't be the target of spells or abilities your opponents control.)