Back on dry land...

Right near the Blue Mosque is the Hippodrome, the former circus built by Constantine on the sight of Byzantium's (much) earlier hippodrome. Incidentally, the bronze horses that adorn St. Mark's in Venice were plunder from this former racecourse. Today, it's a broad open plaza with plenty of strolling tourists. Its main feature is part of an obelisk which originally belonged to Thutmoses III (1490 B.C.), brought here from Karnak in 390 by Theodosius the Great. It's in damn fine condition for something nearly 3,500 years old, now perched on a marble base carved to the glory of Theodosius (first photo from 1990):

Another ornament of the Hippodrome is the Serpent Column, cast to celebrate the victory of the Greeks over the Persians during the Persian Wars in the 5th century BC and moved here by Constantine from the Temple of Apollo at Delphi. You can see how much the ground level has changed since then.

 There's a lovely old building on one side the Hippodrome that I particularly admired - no idea what it is: