İDO  provides fast ferry services between Istanbul's Yenikapı jetty close to Sultanahmet and Armutlu. They depart at 6:45PM every day (as of April; possibly more frequent during summer months) and cost 17 TL pp. It takes around 1hr 20min to get to Armutlu. These direct fast ferry services to Armutlu are for foot passengers only. Those willing to drive can shorten total driving time by taking car fast ferries to Yalova from the same jetty, several times daily, which cost 75 TL for the car and 12 TL per each passenger occupying the car. Drivers are exempt from an additional fee other than the car's.
The road approaching from east (Gemlik) is in good condition, while the road coming from north (Yalova) is very curvy, so even if you are starting in Yalova, it may be better to take the trouble and go for the slightly longer de-tour through Gemlik.
There are hot springs (kaplıca) around the town.
(+90) 226 is the telephone code for the area.