A Visa is indeed required in Turkey but it's very easy and affordable. If I remember right it's about $40. and you don't have to make arrangments in advance of you trip unless flying. The ship will have a blanket Visa and some lines cover the cost, some charge the cost per person onboard the ship. Even if they cover it, it's in your cruise fare so either way you're paying it.
I think your T/A may be confusing the regulation of Russia and some other Visa required destinations with the simple and easy Turkish Visa. In Russia it's true that you cannot go on your own withour an individual Visa and it's pricier and must be processed in advance. Nothing to worry about in Turkey.