We 'did' the Panama Canal last April. Initially, we thought we might need a yellow fever vaccination but it turned out not to be the case. We were given the same advice as Blyth and at no point on the cruise were we asked for proof of vaccination against yellow fever.
Cruise company employees are generally underpaid call-centre staff! They are not qualified to give medical advise. Generally if the company needs a medical cert proving innoculations then they will know, but the only company I have ever heard of that does require this is HAL. Remember that you are on a US santitised cruise ship, and not treking in the jungle!
We are doing the panama canal and as I work in General Practice asked our nurse to have a look at the travel website - it said that yellow fever is sometimes recommended. You only need proof if you are coming in from an infected country. We also need Tetanus, Hep A and Typhoid. We are also going to need Malaria tablets. It annoys me that travel agents don't tell customers that they may need vaccinations when they are booking exotic trips at short notice. Some patients go away not covered. (Just a gripe of mine).
The advice we have been given is you are not staying for any length of time on shore so don't drink water that isn't boiled and no ice in your drinks and other simple precautions but no need for any vacinations so far, but best check the World Health Organisation webpage to see if there is something new, or consult with your local GP at least 6 months before you travel. I don't think you are asking if the nurses on the ship would deal with this but your own within your GP practice ??