A 200 is pretty marginal for most wildlife but I think that Todd Caswell (who contributes here pretty regularly and has some spectacular images) uses a Canon 70-200 for a lot of the dog photos he posts. On the other hand, I know that Tony Zappia (another contributor here) uses a 300 F:2.8 for much of his dog photography. The F:2.8 is a salient point here. A 300 is pretty much at the lower end of what one would use in most wildlife photography and as most wildlife photography is shot around sunrise and sunset, having a fast maximum aperture is a real consideration, but one pays a hefty premium for that additional speed. At this point, my recommendation would be to go to KEH.com and to Robertsimaging.com and search their previously owned departments for the 70-200 VR lens first (give them a call also, as stock turns over pretty quickly sometimes). The 70-200 MAY be capable of capturing images of ducks dropping into the spread, and will certainly be great for getting photos of your dog returning with birds or when you are able to set up photos of your dog working or just posing. Also an excellent lens (although a bit heavy) for photographing candids of family and friends, etc. Then when you are ready, same deal for the 300 and by then you might be able to consider a previously owned 300 F:2.8, but the F:4 should also be a very nice lens, just a bit more limited in low light conditions. If you are used to using a "kit" lens, you will be amazed at the optical quality an build of the pro lenses, and shocked by the weight. If the Nikon 70-200 is anything like the Canon, it is going to feel like a boat anchor at first and will certainly be a bit more cumbersome than what you might be used to, but well worth it. The 300 F:4 will be lighter, and the 300F:2.8 pretty heavy. Once you upgrade your equipment, you might want to get scheduled all hazard insurance. The insurance is pretty reasonable and I know more of more than one camera that has ended up in the drink, and that will really kill your day.
I just went back and looked at the images you posted and if the one of your jumping dog was taken at 55mm, the 70-200 will enlarge that puppy approx. 3.6 times. That will be pretty close.
HPL