We do this all the time. Mainly because we want to do a site visit so we can see the resort and get as much information about it as we can for our clients who may be interested in staying at the resort. But we also like to get away from the crowds and spend a day at a nice resort. Alot of times we've been to the port so many times, there's nothing we really want to do, so we take that opportunity to check out some of the resorts in the area. Works great because there are definitely some resorts we would suggest and others we would not.
While there are some companies in some places who sell various day passes, like in Cozumel for an example, it's usually best to contact the resort directly. Depending on where you go, we see prices ranging from around $45 to about $75 per person, but that usually includes all the food, beverages, and water activities for the day.