So I did some number crunching to find the real benefits(or not) of using Healing Wave as opposed to Lesser Healing Wave. I won't bore you with all the actual math, mostly because I don't want to type it out and partly because it is very basic math. The only thing to take into account when looking at numbers like these is that they change with totems, trinkets and buffs, so my numbers will be vastly different from yours.
+343 in mana use
+5636 in healing done unbuffed-2.25 second cast w/o Tidal Waves
- 1.57 second cast w/ Tidal Waves
+6182 in healing done w/ Totems-2.14 second cast w/o Tidal Waves
-1.5 second cast w/ Tidal Waves
+9362 in healing done with Crit
-343 in mana use
-5636 in healing done unbuffed- 1.35 second cast
-6182 in healing done w/ Totems- 1.3 second cast
-9362 in healing done with Crit
And there it is, the real numbers between Healing Wave and Lesser Healing Wave. Please take into account buffs, totems, trinkets and the like before saying I'm incorrect.
I myself use the riptide + Healing Wave combo, with Lesser Healing Wave and Chain Heals thrown in if needed. I find myself at around 60-70% mana at the end of a fight unless it was raid damage heavy or just very intense. This method has a very high healing output when you add Glyph of Healing Wave and Ancestral Awakening, and a higher mana return, in most cases, with Improved Water Shield.