Sumahan is one of our favorite boutique hotels. Located directly on the Bosphorus on the Asian side, it has a superb view down the waterway to the old city of Istanbul. Plus you get to watch all the boats going by and the sea birds as well as dolphins playing out in the water (we spotted a group of dolphins three or four times during our stay).
This is a small hotel - only 13 rooms, I think. It used to be 24 rooms when we stayed there previously, but the downturn in tourism has forced the owners to rent out one of the buildings. Still, it's a gem. The room we were in was on the second floor of the main building (actually the level you walk into from the street, as the building is set into a small hillside), and faced the water, as do all the rooms in the hotel. It had big windows that could be thrown open to enjoy the fresh breeze and sights and sounds of the Bosphorus. I think all the second floor rooms are similar. The first floor rooms are at water level and have a short lawn between their French doors and the water, with a couple of garden chairs and a small table set up for each room.
The rooms are very comfortable - ours had a king bed, which was set up on a platform about one foot high (remember this if you get up in the night), and the bed as set at a slight angle in order to take advantage of the view out the windows - you can lie in bed and see the water. At the foot of the bed is a wide easy chair and ottoman that makes for great relaxing & viewing.