The quickest and easiest way to go to Yalova from Istanbul is to take a fast ferry from one of Istanbul’s many jetties. The major jetties with a connection to Yalova are located in Yenikapi (south of Sultanahmet), Kabatas (near Taksim) and Bostanci (Asian side). The voyage doesn’t take more than one hour.
There are some tea gardens by the shore where you can also eat balık ekmek (a fish sandwiched inside half a bread).
The telephone code of Yalova is 226 (+90 226 when calling from out of Turkey). It used to be 216 prior to late 1990s, so old guidebooks may say so, which is not useful any more.