It gains an additional instance of the Shadow keyword. Per the rules for Shadows: "If a card has multiple instances of the shadow keyword, each with a different (X) value, the player controlling the card may choose which instance of the shadow keyword is used to bring the card out of shadows."
King’s Landing does NOT permit a player to marshal a location into shadows. King’s Landing allows you to “marshal a location” from your discard pile, and marshaling a card into shadows is not considered to be “marshaling a location.”
Water Gardens will not reduce the cost to marshal anything into shadows — basically, anything that affects the marshaling of a card with a certain characteristic will not affect marshaling a card into shadows. The game will look at the card being marshaled into shadows and see that characteristic as undefined (i.e. a card being marshaled into shadows cannot be defined as either “character” or “non-character” — it’s just a card being marshaled into shadows).
Water Gardens does reduce the cost of playing an event out of shadows. Playing an event from shadows is considered to be “playing an event,” and follows the same initiation process as playing an event from hand. By the time the process reaches Step 3 “Apply any modifiers to the cost,” the event can be defined as a “non-character card” that is being played and is thus eligible for the discount.