Need to hang 60 lb water heater on exterior stucco wall
Yes- there are toggle bolts that can support 60#. The real issues is the stucco wall. Plus in earthquake area, the wall-mounted water heater should be mounted on solid support members.  

A better way to mount the water heater is to have two horizonal wooden spans that are bolted to the studs behind the stucco wall with the proper size lag bolts into the studs.  The purpose of the second span which is below the first spand is to provide support to the bottom section of the tank since it is now 1 1/2" from the wall. The water brackets are screwed into the top horizontal span.
And alternate method is use a 3/4 plywood panel, the width would span the two available joist and additional width to where you want to hang the water heater.  The plywood would be mounted to the at the top and bottom of the plywood plate on the studs.  Brackets of water heater easily screwed into the plywood sheet.
My preference is the 2x4, because your screws for the bracket will have more backing to screw into.  With the plywood plate, only 3/4 of holding power.
Yes there are toggle bolts that will hold the weight, but would not recommend this in stucco for a suspended water heater.