Ill jump in here to say this. Pacmans or frogs of the family Ceratophrys are terrestrial. They spend 90% of their entire life on land burrowed in the forest floor. If they were anywhere near aquatic or semi-aquatic they would live and spend most of their lives in water which they do not. Their behavior and life-style is very simiar to that of toads which only go to water to breed as do Pacmans. They need substrate to burrow in thus is their natural habitat and behavior. To feel secure they need this. They do not sleep in their water dish. They like to soak but do not like to be in water all the time. I have 2 Cranwellis and neither use their water dishes almost at. All. Grif's is quite wide and could almost fit 2 of her in it and she is quite large. There is a reason why Pyxies are aptly named African (BULL FROGS). They are Semi Aquatic and spend 60/40 land and water so it doesn't affect them as much.
Just because you found random photos of a pyxie in grass and an ornate pacman in water does NOT justify nor prove that an all water setup for a Ceratophrys frog is the way to go. They don't live that way and so it is not and ideal setup for one to live in.
Yes they do have similar shape/have teeth/burrow/and puff up with air but are in no means actually similar. They have a completely different habitat and lifestyle. Just because one frog was in the water a lot doesn't mean another will like it. Also your frog swimming with its head under the water was probably due to its feet slipping on the bottom of the tank and it leaning forward because of it not being able to gain traction. I've seen my do this. They are horrible swimmers. Plus if they were meant to be in water their nostrils would close shut like aquatic and semi-aquatic frogs do. Their do not.