NTSB Document NTSB-AAS-72-3, dated 1972 but don't see why things will have changed.
Emergency Landing Techniques in Small Fixed Wing Aircraft
PDF
http://amelia.db.erau.edu/reports/ntsb/aas/AAS72-03.pdf
On page 7 (page 12 of 14 in the PDF) you will find
A well-executed water landing probably involves less deceleration violence that a poor tree landing or a touchdown on extremely rough terrain
Plenty of other good information in there.
Obviously you wouldn't aim for the middle of a lake where you might drown, you would ditch close to the shore. Also, quite a few lakes have beaches alongside, and quite a few reservoirs are not full and have shorelines which are clear of vegetation. As always, it depends, but if over nasty terrain with nothing like a road around I'd head towards a lake.