Why are emergency water landings the majority of the pre-flight safety briefing if they're so rare?
The entry doors to an aircraft that is required to have life rafts are equipped with "slide rafts," which are dual purpose (land/water) so there is not any material difference in the weight. Some aircraft will also have perhaps one (maybe two) rafts in the cabin overhead (depending on the need for extra life raft capacity not covered in the entry door slide/raft).

I imagine, if controlled water landings were more frequent, design and technology might be modified in some way to make successful evacuations more efficient if that were necessary.  But, as you point out, controlled water landings are relatively rare, so likely the cost/benefit of creating an entirely different fuselage/wings/evac system would not be justified.

Of course, the fairly recent successful controlled water landing and evacuation into the Hudson is ample justification for having personal flotation devices (life vests, etc.) and associated emergency equipment designed primarily for water survival. Also, the design of that aircraft apparently provided for adequate flotation and evacuation as can be seen from the photographs.   

The safety briefing before each flight by the Flight Attendants is relatively short and maybe 60 seconds are dedicated to explaining the operation of emergency equipment specific to water landings.