I used to start greenies that way whenever I could. It's a lot less stressful for them to use the water to help you when you mount, rather than throw a leg over. The water will support some of your weight and that's less of a change for the horse to deal with, even when they have been groundworked extensively and are used to weight leaning over them. Most of mine would be perfectly happy with me being up there by the time they walked out of the pond, and never gave thought to how to get me off. I never had one put up a fight with this method, though I have had some issues with first mounting in a round pen.
"Keep a leg on each side and your mind in the middle"