Can't "Ти добре плаваєш?" also be correct?
It is correct. I think it even sounds better :)
I agree. Sounds better.
Ви добре плаваэте?
Also, "Чи ти добре плаваєш?" & "Чи ти плаваєш добре?" are also correct forms of speech, but were not accepted.
("ЧИ ти...?" literally means "DID you...?)