The Sea of Marmara as seen from the Küçük Ayasofya neighborhood in Fatih. Some of these ships are probably waiting their turn to navigate the challenging Bosphorus. For details about how treacherous this can be, here is information about navigating your yacht through the straights.
A north-eastern view of the Sea of Marmara as seen from the Asian side of Istanbul.
Here we see the Sea of Marmara from the northern-most tower at Yedikule Fortress (Yedikule Hısarı). Yedikule, which means seven towers, was built in 1458 as an addition to the Theodosian Walls which were built in the 5th Century. Yedikule is at the southeastern edge of the old city. You can see the fortress (‘from space‘) if you are zoomed in close enough on this map. A short description of Yedikule Fortress can be found here; a slightly longer version, here.