I've never had a need to enable the international plan in Puerto Rico. Most of the island is part of the Nationwide and/or America's choice coverage area. Using my phone in PR has always worked as if I were in the states. Beware, there are some spots in the island that you could end up roaming. Roaming in those areas is .69 a minute.
You said you are on a cruise so you will be in the Old San Juan area (near the port). That entire area is covered.
Under Additional Roaming Details, it states: "Most of Puerto Rico is included in the Nationwide and America’s Choice Coverage Areas. If you roam out of the Nationwide or America’s Choice coverage area, you will be charged $.69 per minute. Please use your device's banner as an indicator of your roaming status. Click here to view the approximate coverage areas."