For a fun cocktail cruise, I will often drive my boat up the river. It's a comfortable ride and the boat does well at about 3/4 throttle setting. Every now and then, I will turn around and go back down the river. I've never noticed a change in boat speed when I make this turn, nor have I had to increase throttle setting to maintain boat speed throughout the turn, no matter how fast the current is.
I do notice a large change in speed relative to the land on shore, but absolutely no change in driving characteristics relative to the water that the boat is driving in.
I've also never noticed water starting to come up over the transom as the boat slows down in this turn, or that the boat starts sliding backwards due to having a low inertia relative to the land on shore. Really, as I sit there with my cocktail, the only inertia that matters is the inertial relationship to the flowing water, even though someone sitting on the shore might see something entirely different.