Once he goes to the radio tower to disable Rousseau's distress signal. Juliet Burke (Elizabeth Mitchell) informs Sayid that this will not work because the underwater "Looking Glass" is blocking outgoing transmissions.
After he travels to the radio tower to disable Rousseau's emergency beacon, Juliet Burke (Elizabeth Mitchell) advises Sayid that this plan will not succeed because the underwater "Looking Glass" is impeding outgoing communications.