Stormtide Leviathan

Stormtide Leviathan's second ability causes each land on the battlefield to have the land type Island. Each land thus has the ability "{T}: Add {U}." Nothing else changes about those lands, including their names, other subtypes, other abilities, and whether they're legendary or basic.

Stormtide Leviathan's third ability affects all creatures with neither flying nor islandwalk, regardless of who controls them. They can't attack any player or planeswalker.

Islandwalk cares about lands with the land type Island, not necessarily lands named Island.