Forgive Arthur for flailing around, even after Prussia gets a hold of him. He is a fire dragon, gets his powers from the sun or hot weather. Not only is it night-time, but now he's in cold water. So whatever powers he has is minimal.

He doesn't answer Prussia because he's starting to change back. Moments later, Arthur is a full dragon again, and he's taking to the skies with Prussia on his back. So you said you wanted a ride...