Needless to say, Istanbul is one of the most romantic towns in the world, especially if you explore the city by boat.
A starter is a Bosphorus Cruise if you have not done it yet. The cruise with the regular Istanbul municipality ferry which starts in Eminönü ends in Anadolu Kavağı, where you can have lunch and even do a short trek above the small fishing village to the Genoese Castle overlooking the Black Sea and the Bosphorus. The vistas are wonderful, with a blue sea extending over to Odessa, Ukraine. You will see ships going to Black Sea countries like small whitish dots on the blue sea. And if you turn around, you will see the strait of Bosphorus in all its grandeur lying to your south.
Eat fried mussel sandwiches if you are into seafood — especially good during winter and spring. Do the bargaining for the fish before you enter any fish restaurant.
You can also visit the beautiful Princes' Islands, the biggest and most beautiful being Büyükada (Big Island). Since traffic is not allowed on the island, you can take a horse buggy ride; you should negotiate the fare before doing a tour. There are two types of tours: the big and the small. Here too, the specialty is fish. There are also fried mussel shops and ice cream parlors.
There are two types of ferries in Istanbul. The ones that are typical to Istanbul are the landmark boats whose design is taken after the British boats built for Constantinople, and although that design has evolved, the look is similar. The second type are Australian-designed ferries called seabuses that look like airplanes on the sea once inside.
The major difference is the outer passenger space of the former traditional ferries. They have many areas on the sides, at the top, and at the back of the boat that give you the ability to travel outside like on a cruise ship — to feel the breeze, take pictures, hear the waves, and feed the seagulls with simit, a typical Turkish fun. You will see seagulls following the boat at the same speed for a while, so they will look as if they are hanging from the sky by some invisible cord.
Instead of the ferry, you can take a private boat tour, but they do not go that far up the Bosphorus, so the trip is shorter. However, a private boat trip at night is advised, as Istanbul has become more beautiful due to the great lighting of many landmark buildings on the water.
An important note: you cannot charter a boat to cruise from Istanbul to the southern coast. You need to take a ferry that departs on Fridays to Izmir and then on to Bodrum. It is not time-efficient, but if you like sea travel, that could be a good trip to a southern Turkish resort. You can charter a gulet once at Bodrum or coastal towns in the southern Aegean.
Also check Ferries from Istanbul to other cities and countries, or back to Istanbul Travel Guide.