Hellenic Seaways offers hydrofoil (1 hour) and catamaran (1 hour 20 mins) service from Piraeus daily, with between two and four sailings daily. Check schedules and book tickets here.
Saronic Ferries offers a ferryboat service via Aegina and Methana daily in summer, weekends only in winter. The journey takes between 2.5 and 3 hours. Check schedules here.
Alternately, you can reach Poros by car. It’s a 2-hour drive from Athens to Galatas, then a 10-minute crossing by car ferry (last sailing varies by season) or water taxi (24 hours).