One of the keys to take action shots is to be ready to click, and clicking, before things happen. Your mind has to be set. If you are just seeing the image for the first time in the view finder, you are too late.
So great job capturing the moment. Good sign of action instincts.
Where you need to work is the technical aspects of the picture. Focus issues - which is normal with action shots. This could often be an issue with the lens you are using. I have a cheapy telephoto lens that does the same.
Composition of the overall shot makes it look like a snapshot. As stated, getting in closer to the dolphins to have them fill the frame would add alot more to the shot. But as they are not in focus, doing this in post processing (crop) might not be the best.
Horizon is slanted. I try and look at the various focus points in my view finder and use the left most and right most ones as guides to help level the picture.
Overall, great start! Keep up the practice.