Located northwest of Poznań’s Old Town, and just northeast of the airport, Lake Rusałka is perfectly accessible via public transport, but getting there usually involves a complex cocktail of trams and buses and obviously depends entirely upon where you are and which part of the area you want to explore. As such, taxi is definitely your easiest option, and a ride out to the western suburbs should only set you back between 15 and 25zł depending on where you are in the centre. Nonetheless, here are your best options for visiting key points in the area via public transport.
To the Botanical Gardens and south shore:
The 'Ogrody' tram roundabout is just outside the entrance of the Botanical Garden. At the moment the tramline is under construction after Rynek Jeżycki, but bus T2 makes the same route. Take tram 17 from 'Plac Wielkopolski' and change to bus T2 at 'Rynek Jeżycki,' getting off at 'Ogrody.' From 'Ogrody' it is also only about a 20min walk up ul. Botaniczna, across ul. Wawrzyńca, and to the trail on the lake's south side.
To the north shore beach area:
Catch bus 60 from 'Małe Garbary,' getting off at 'Golęcin;' or bus 64 from Rynek Jeżycki, getting off at 'Golęcińska' (one stop further than 'Golęcin'). It's a 15-20min walk through the woods from either stop, though "Golęcińska' is a bit closer.