Yalova is easiest from Istanbul by fast ferry. Boats run from Yenikapı on the European side and Pendik on the Asian side, landing close to the centre of town. The crossing is part of the appeal, especially if you are using Yalova as a low-effort break from Istanbul.
By road, the Osmangazi Bridge and O-5 motorway have made Yalova easier to bypass than before. Buses connect the city with Istanbul, Bursa, Izmir, and other Marmara routes, but for a short visit the ferry is the cleanest arrival.