We have labs and an inflatable
dinghy. Our late 90lb Lab Colby and our current smaller (65lbs) lab both developed the same technique for boarding the
dinghy from our sailboats (Catalina 30 and Bristol 38.8).
They approach the boarding area of the sailboat and jump onto the pontoon of the inflatable and then into the dinghy. They both do it as if it is one smooth motion with two steps. They either land on the bow chamber or the pontoon lined up next to the sailboat. You almost need to have the dinghy about a foot or so off of the boat so that they don't have to jump directly down.
We also use a life jacket on the dogs with a handle. If you hold the dog from the dinghy by the handle and guide them off of the larger boat and onto the dinghy, you can prevent any hard crashes.