At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land they control of their 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.
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).