"Where do you swim?"
The question "Where do you swim?" can be translated these ways:
Hol úszol? (2nd person singular)
Hol úsztok? (2nd person plural)
Hol úszik? (3rd person singular as formal you singular)
Hol úsznak? (3rd person plural as formal you plural)
So does that mean "Hol úszik" should have been accepted, even though it is formal?
If you like to add the pronoun, it would be:
Ön hova úszik? or Hova úszik ön?
At the start of this course with nearly all the you-sentences you had to guess, if the 2nd person singular or plural will be accepted. Now those two form are mostly accepted as it seems, for the formal form is still not in a lot of cases.
Shouldn't the ending be -sz, since it's you singular, ie. úszolsz? Or is this an irregular of some sorts?
úszik (he swims) úszol (you swim). All verbs whose stem ends in a sibilant use -ol, -el for second person.
An additional hint:
in general ALL ik-verbs use "-ol/-el-/öl" for the second person.
The best example: lakik => te lakol (though a lot of people say "laksz")
This is very old fashioned. My 1950s book gives this - but more recent grammars use "laksz" - and that is what is taught at University (and by Akadémiai Kiadó). In Hungary they actually release changes to the language - the lakol -> laksz must have come out pre-1999.