You can't very well make a day trip to Celestun from Uayamon, but you can from Santa Rosa. (Unless someone gives you good reason to do so, do NOT take the back road west out of Santa Rosa. It could be completely washed out, and if your car broke down, you might be stuck for a long while. Instead, go up to Merida and then west to Celestun.)

Did you determine a non-SPG hotel that you would like in or near Celestun? I'm not sure if Celestun's beach is what you are expecting. It does have the beach, but it's main attraction is the back side of the island's (or is it a peninsula?) flamingo colony and wildlife reserve.