First of all let's have a look on fly casting instructor exams. I think the best place to run them is precisely where most of todays teaching takes place. Not long ago I asked a large number of well respected fly casting instructors located in numerous different countries throughout the world where most of their teaching took place. The main (80%) answer was very straight: "On grass". Thus it seems to be a proper choice to run a fly casting instructor exam on grass either.
Personally I taught a lot on both grass and water. For me grass always provided some significant advantages from the teaching (and learning) perspective:
1) A still can be layed out and then explained in detail as long as needed. On water the line often drifts away pretty fast.
2) In teaching there are some important points I want to take care of. That is having the sun in the student's back. The wind should be on a side make casting comfortable for everyone. When demoing a cast - depending on what to focus on - I choose either my line hand or my rod hand being between me and the student for best visualizing. Of course having proper contrast with the surroundings to make the loops best visual is important as well. All this usually is much easier to get perfectly aligned on grass!
3) Grass takes away much less concentration compared to water. The worst place for teaching fly casting is a fast running river full of slippery stones and loaded with jumping fish. Having the student concentrating at one key point at a time gets very tricky here!
4) On grass there are usually no bushes, no trees, no waves and simply no water catching up the student's fly and/or line when not having proper casting available yet. Thus it's fair to summarize grass to be more forgiving (smaller casting issues). In my experience this helps to get faster into proper fly casting.
5) A proper place for fly casting on grass can be found nearly everywhere. To find a proper water easily might be linked to driving quite a distance for many of us.
About the difference...
I yet have to come across a first student who can perform a proper roll cast on grass which then wouldn't work well on water, too. In my book the key points to perform a proper roll cast are the same.
In the end it has worked best for me to combine both teaching on grass and finishing the lesson on water. But yes, I especially prefer to start out on grass - both in single hand and in double hand fly casting that is!
So where ever you are going to train your fly casting or maybe meet a fly casting instructor I hope you'll have a lot of fun by increasing your own level pretty fast. That's what happens when all aspects come togheter in a proper teaching venue.
All my best
p.s.: Spey casting obviously can be done in nearly every place on the planet, even in my living room. But this was highlighted here already.
Some pictures from last days...