Hi,
On the 747's that I have flown (BA & VS) the emergency exit slides double as life rafts ( according to the safety card). In the event of a ditiching once you are on baord you separate the slide/raft from the aircraft and off you go.
At one per exit (not sure if the overwing exit slides can be used as rafts) thst makes it 8 or 10 on a Boeing 747-400, 8 for the A340-300.
Regards
TBS