I assumed it was a matter of the dog desiring to go on a straight line directly towards the ball, and knowing that getting as close as possible by land will use

I assumed it was a matter of the dog desiring to go on a straight line directly towards the ball, and knowing that getting as close as possible by land will use less energy- but as he proceeds along land, his desire to head directly towards the ball increases due to anticipation, until eventually the force of anticipation grows larger than the resistance to excess effort. I bet as a puppy, he went in a straight line and ignored exertion, and as an elderly dog he'll minimize swimming time/effort, and as a middle-aged dog he's splitting the difference.