You may try contact AWF Kraków club:
azs-awf.pl/content/view/522/6/ - tel. 793 666 981, firstname.lastname@example.org
Although on the ad they invite people (kids and adults) with a disability to the rowing section, on their FB page they announce the recruitment for youth 12-16 years old for training.
facebook.com/AZSAWFKrakow/ - tel. 605 979 104, email@example.com
It's always worth to ask.
UKS 1993 - this is also a recommended rowing club in Krakow with ergometers:
30-105 Kraków ul. Kościuszki 38 skr. poczt. 153, phone +48 501 426 091, Fax 012-6449797, e-mail: firstname.lastname@example.org; UKS93@interia.pl
The last one I found:
Skarbińskiego 10/117, 30-071 Kraków, tel. 12 658 16 85, e-mail: email@example.com
I'm a UK rower visiting Krakow. Just been for a jog along the Vistula on a beautiful warm, calm morning and astonished not to see a single rower on the most perfect water I've seen for a long time. Where are the rowers! :-D