I went on a short boat excursion yesterday. I took the boat from the boarding point by Galata Bridge. There are multiple cruises leaving all the time, you just pay and hop on. I chose “Bosphorous Everyday Cruise” because the boats are quite small, maybe with seating for 120 passengers. Another company runs much larger boats, which look like they take several hundred. My cruise cost TL 250, about $5.00, and lasted for one hour.

To be honest I didn’t see any of the historical sites depicted on the placard, which I thought included the Rumelihisarı fortress, but I might have misread the poster, or that might have been for a different cruise. Not to worry; it was a really nice cruise. I took a lot of photos; cruise liners, an oil tanker travelling down the Bosphorous from the Black Sea to the Aegean, the lighthouse, and small boats bobbing up and down. And, the Bosphorous Bridge itself.
The boat itself was comfortable, with plenty of good viewing. You could buy drinks and snacks, or have your photo taken with a parrot.
Staying in Istanbul can be quite expensive. A double shot Turkish coffee, with a (delicious) croissant and a spinich, (in fact sea-weed), and cheese slice, set me back about $12.00 the other day. So, a $5.00 cruise on the Bosphorous is money very well spent.
