Hi, Xel-Ha from Cozumel is 45min ferry to Playa del Carmen, 45 min bus drive to Xel-Ha and the same back, a 5 hour tour would only allow 2 hours at Xel-Ha (which sometimes are compromised to to being high cruise season and international and national holidays!!), which I personally consider very limited due to the many attractions and activities, which are really worth spending some time on. The regular Xel Ha tour is 8 hours and allows ca. 4.5 hours in the park, I would consider this a good amount of time to spend at this site as well as at Xcaret (only 15min from playa del carmen). Doing it by yourselfes is not difficult, but requires some information. Take a taxi to downtown fiscal pier, you got a ferry every hour (with some exeptions, ask the operators of the ferry and take notice there are two companies, asking one only they will only give you their times and not the competition ones!), then a taxi or collectivo ride to Xel-Ha, that is it, just make sure you know when to take the ferry back to Cozumel, in the worst case, take a 10min plane ride back to Cozumel (ca. 150 US). Xel-Ha is a very special and beautiful and secure natural park/lagoon, as Xcaret, a must do in a lifetime.
Hope it helps!