Yes and no. If it's "Friday", which is only one day, they mean the same thing.
But if you had "w każdym miesiącu", it just means that there is no month when you don't swim. So let's say that you swim three times per week regardless of the month. And "co miesiąc" would mean that you swim on, let's say, 17th October, and then 17th November, 17th December, etc.
pływać: imperfective, habitual/general, multidirectional
płynąć: imperfective, continuous, unidirectional
Am I right? Also, when I search on Wordreference, I see several variations of both verbs, such as "popływać," "dopływać/dopłnąć." They all describe these verbs as "moving through water" and give no details on their usage. Could you please explain these to me if it's not too much trouble?