No special access issues
JT-lines operate from Eira (Cafe Carusel) AND Ruoholahti (next to Restaurant Faro) once per hour. Return tickets costs 7,5e. (2020 starting from June). With your own boat its open all year.
https://www.hel.fi/helsinki/fi/kulttuuri-ja-vapaa-aika/ulkoilu/saaristossa/pihlajasaari