One thing I learned a long time ago when working with aquatic species is to get low and shoot wide. Getting low helps to make the subject look bigger and as well as making it easier to hold the subject closer to the water, thus helping to reduce the amount of time the fish is out of the water. Remember the goal is to get the shots quick for a fast release. The wide angle helps to exaggerate proportions while also dramatically changing the focal plane when shooting wide open. The results can be really fun.
Images Captured with Nikon D750, 18-35 f3.5-4.5, on Lexar UDMA Digital Film