Translation:He swam for one hour today.
This is a part of the "separable" class of verbs in chinese. what's actually going on here is the verb 游 is in a family of verbs that don't make sense unless they're followed by an associated object. 泳 is the "dummy" object that fulfills this role to make the concept make sense (we also see this with verbs like 吃, which is usually followed by 饭 unless something else is specified, and 看, for which the dummy object is usually 书) . following typical chinese grammar, the object must come after everything time- and place-related in the sentence, and so the "verb" is separated.
Hopefully that makes sense.
As I understand it, it is the verb part of the verb+object construction that has to be qualified, so both the 了 and the duration have to come after the verb and before the object.
It is a separable verb (or a verb object phrase)