Does a land enchanted by Spreading Seas lose its original land type immediately?

Spreading Seas is an Aura that gives the enchanted land the subtype Island and makes it lose its other land subtypes. This change happens as a continuous effect the moment Spreading Seas becomes attached, as part of the game's layer system (specifically layer 4 for type-changing effects).

Under CR 305.7 and the layer rules (CR 613), type-changing effects apply in layer 4. Spreading Seas replaces the enchanted land's land subtypes (Plains, Mountain, Swamp, Forest, etc.) with just Island. The land retains its supertype (e.g., Basic or Legendary) and its card type (Land), but all of its previous land subtypes are gone immediately.

Because the land is now an Island, it produces blue mana instead of whatever mana it previously produced. The land's printed text about producing other mana is effectively overridden by the type change — Island has the intrinsic ability to tap for blue mana (CR 305.6).

Concrete example: Your opponent controls a Stomping Ground (a Mountain Forest). You enchant it with Spreading Seas. Immediately, it stops being a Mountain and a Forest — it is now only an Island. It no longer produces red or green mana; it taps for blue mana instead. Any effects that care about Mountains or Forests no longer see it as such.

Note: If Spreading Seas leaves the battlefield, the continuous effect ends and the land immediately regains its original land types as printed on the card.