Are evacuation slides meant to be used as safety boats?
The inflatable slides are indeed also life rafts and they are meant to be detached. Consider United 1549 (landed on the Hudson). Here you can see the slides inflated and floating



Here you can see people in the slide, which has been detached from the airplane


The Wikipedia article mentions it specifically


  Sullenberger advised the ferry crews to rescue those on the wings first, as they were in more jeopardy than those on the slides, which detached to become life rafts.


It's impractical to keep an airplane afloat in the water. Assuming you successfully land intact, the plane is still a metal tube on the water. It may be airtight, but water will eventually begin to invade. Airbus has a water ditch button to help slow (but not prevent) flooding


  Some aircraft are designed with the possibility of a water landing in mind. Airbus aircraft, for example, feature a "ditching button" which, if pressed, closes valves and openings underneath the aircraft, including the outflow valve, the air inlet for the emergency RAT, the avionics inlet, the extract valve, and the flow control valve. It is meant to slow flooding in a water landing.


You want to evacuate as soon as possible. Remember, the slides help keep some water away from the door, but the plane will eventually flood and sink.