My German boyfriend is flying into Tampa (direct from Frankfurt) to visit me. From there, we are taking a cruise from Tampa, to Cozumel, to the Grand Cayman and back to Tampa. Does he need a special visa other than the ESTA visa he always gets when he travels to the US? I am trying to avoid having him run into some sort of immigration issue when we get back to Tampa.
Typically, cruise ships get a special treatment in the sense that you are never considered to have 'entered' the destination countries. That takes care of any potential issues in Cozumel, Grand Cayman, etc. You need to have a valid passport to get on board of the cruise ship, and then you are allowed to go on land temporarily with the cruise ship boarding card, and that's it. You can (and should) leave the passport aboard.
When you come back to Tampa (or any other US port of entry), you will be processed identical to immigration at the airport (although it is much faster...). You need to have (again) the same identical paperwork and visas you would need when flying into any US city. In your case, as a German citizen with a still valid ESTA, you only need to have your passport with you, same as when originally arriving at the airport. Note that customs applies exactly like at the airport too, so don't bring a gallon of rum or such.
[I am German citizen and have made many cruises from Florida into the Caribbean]