Prague this Sunday

Yes, there are some freediving clubs and plenty of freedivers. Try contacting people from potápìní - Deep Blue , from Freediving , or from AIDA - AIDA CZ - freediving, nádechové potápìní A more complete list of Czech freediving clubs is at clubs czech @ APNEA.cz
There are also over 40 Czech freedivers subscribed at apnea.cz, and since you are member too, you can contact them though a PM or post to the forum there (though the forum is very quiet - I think there are no other posts than my own ones, so it is not too probable someone will see it there). However, some people from Prague do peek in here sometimes too, so they may reply. I'll be in Prague just the week after that.

As for night life - there is anything you can imagine (or almost). Plenty of clubs, concerts, pubs. A pint of excellent beer costs between 0.50€ and 1€ in a pub, possibly slighly more expensive in the tourist center.