I want to understand why, if I enchant, let's say Clifftop Retreat, with Spreading Seas, does it only produce what an island would produce (1 blue mana) and loses all other abilities. Then I have Moonring Island, which is also an island, but with another ability.
305.7. If an effect sets a land's subtype to one or more of the basic land types, the land no longer has its old land type. It loses all abilities generated from its rules text and its old land types, and it gains the appropriate mana ability for each new basic land type. Note that this doesn't remove any abilities that were granted to the land by other effects. Setting a land's subtype doesn't add or remove any card types (such as creature) or supertypes (such as basic, legendary, and snow) the land may have. If a land gains one or more land types in addition to its own, it keeps its land types and rules text, and it gains the new land types and mana abilities.
In a nutshell, Moonring Island is a land which is an Island in addition to its rules text in the same sense that a Taiga is a Mountain in addition to being a Forest, while the wording of Spreading Seas negates the previous types and abilities of the land enchanted with it. If the wording were, "enchanted land is an Island in addition to its other types," then the enchanted land would retain its original types and rules text, and Spreading Seas would be a considerably worse card.
Your Moonring Island would become an Island. It would have the ability to tap for Blue mana, and nothing else.
It retains its name and Supertype (Legendary, Snow, Basic), but becoming an Island overwrites any abilities it may have had.
Is that true even for Moonring, which is already an Island? I know that Spreading Seas will remove any other types (so Hallowed Fountain will no longer produce white mana), but Moonring is already an Island and only an Island. Since its type doesn't actually change, does it still lose its ability?
Since its type doesn't actually change, does it still lose its ability?
Yes. That's why rule 305.7 says "set." It used to say "change" up until fairly recently (February 2011 to be precise), and then the rule was revised to the current wording to clarify exactly this interaction.
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Please use card tags when you're asking a question about specific cards: [c]Serra Angel[/c] -> Serra Angel.