In my opinion, it's a must do activity. It allows you to observe many tourist attractions from sea. The activity has been described very well by other reviewers here. I would like to share one money saving tip here.
You can do this activity in as little as 10 Turkish Lira per person. Public ferry departs from Eminonu and you can hop on this ferry by paying just 10 Turkish lira per person. Just outside the ticket selling office, you will find people selling tickets at a very large premium. Stay away from these people.
Please note that there won't be any private guidance available on this ferry. For that, you would have to opt in for an comparatively expensive private tour.
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