This is about the safest tour you could possibly do on a cruise. I can't say "totally safe" because that is not true anywhere, but I would do this by myself with no worries. Just use the same common sense you would in a city anywhere in the world, including the U.S.
One suggestion I have is not to carry your valuables in your purse. It is an obvious target if you have the misfortune to encounter a thief. I use a pouch that hangs around my neck and goes under my top. Most of my money, ID's, credit card go into that. I put enough money in a pocket to cover taxi, a soda, etc. My purse is basically a decoy. Just one of my techniques.
Relax, you'll be fine! Have fun.