Artifact, 3 (3)
Rare
At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land he or she controls of his or her choice. That land is an Island for as long as it has a flood counter on it.
At the beginning of each end step, if all lands on the battlefield are Islands, remove all flood counters from them.
Illus. Trevor Hairsine

12/1/2004: The land stays an Island until the flood counter is removed, even if Quicksilver Fountain leaves the battlefield.
12/1/2004: The timestamp of the land-type-changing ability is set when the triggered ability resolves (so each one has a different timestamp).
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern