What qualifies as "overwater"?
I think that 14 CFR Section 91.205(b)(12) defines "overwater". 


  (12) If the aircraft is operated for hire over water and beyond
  power-off gliding distance from shore, approved flotation gear readily
  available to each occupant and, unless the aircraft is operating under
  part 121 of this subchapter, at least one pyrotechnic signaling
  device. As used in this section, “shore” means that area of the land
  adjacent to the water which is above the high water mark and excludes
  land areas which are intermittently under water.


Then 14 CFR Section 121.573 outlines the passenger briefing requirements for flights with overwater operations:


  §121.573   Briefing passengers: Extended overwater operations. 
  
  (a) In
  addition to the oral briefing required by §121.571
  
  (a), each
  certificate holder operating an airplane in extended overwater
  operations shall ensure that all passengers are orally briefed by the
  appropriate crewmember on the location and operation of life
  preservers, liferafts, and other flotation means, including a
  demonstration of the method of donning and inflating a life preserver.
  
  (b) The certificate holder shall describe in its manual the procedure
  to be followed in the briefing required by paragraph (a) of this
  section.
  
  (c) If the airplane proceeds directly over water after takeoff, the
  briefing required by paragraph (a) of this section must be done before
  takeoff.
  
  (d) If the airplane does not proceed directly over water after
  takeoff, no part of the briefing required by paragraph (a) of this
  section has to be given before takeoff, but the entire briefing must
  be given before reaching the overwater part of the flight.


Lastly, the requirement for printed safety booklets is from 14 CFR 121.571(b)  And to answer your last question, it needs to have only information pertinent to the flight, so if the airplane is not going to fly overwater at all, it doesn't need to have it.


  (b) Each certificate holder must carry on each passenger-carrying
  airplane, in convenient locations for use of each passenger, printed
  cards supplementing the oral briefing. Each card must contain
  information pertinent only to the type and model of airplane used for
  that flight, including— 
  
  (1) Diagrams of, and methods of operating, the
  emergency exits; 
  
  (2) Other instructions necessary for use of emergency
  equipment; and 
  
  (3) No later than June 12, 2005, for Domestic and Flag
  scheduled passenger-carrying flights, the sentence, “Final assembly of
  this airplane was completed in [INSERT NAME OF COUNTRY].”