Banking sites anyways encrypt communication, so you don't need to worry about someone else snooping on your communication on the network.
What you do need in order to enhance your security is something that protects your actual computer from intruders lurking on the network. You might consider Comodo firewall, Voodooshield, Hard_Configurator, or other similar softwares. Not necessarily in that order.
VPN is good for privacy. The above-mentioned softwares are good for security.
And make sure you are connecting to the real network of the cruise ship and not to a look-alike impostor.