Lyn is right, you may not have to obtain a visa on your own for Turkey or Russia. I say MAY NOT because many of the cruiselines buy what is called a blanket visa for these ports that would cover you for cruiseline excursions only. If you want to do outside excursions or just get off and see things on your own, you will be required to have an individual visa. Also if you're flying to either spot to begin your cruise (not likely with mainstream lines) or visit before your cruise, you'll have to have the visa to arrive by air. If any of the ports above are on your itinerary ask lots of questions and get info from the Governments website that deals with the info to be sure you have exactly what you need. www.travel.state.gov
is the State Dept. website that has most of the info you'll need and the CDC one for shot info. If in doubt investigate, don't get surprised!