Serenidad Shardon is located in remote Punta Allen. As its owner likes to say, Punta Allen is where people go when they want to get off the beaten track and away from everything. The town itself is a ~35km, 1.5-4 hour drive from Tulum (depending on the roads). There are no gas stations in Punta Allen, so it is wise to fill up before you leave Tulum. Similarly, no one accepts credit cards in Punta Allen (and there are no ATMs) so bring sufficient cash for all your needs. The town generator provides electricity 8 hours a day: from 11-2 and 6-11 pm. There are several small shops that have groceries (at decent prices, despite how far you are from a large supermarket) and a number of casual restaurants that serve very good local fare.
Punta Allen is a home of a sort for the Central American lobster fishing industry. They export their flash-frozen lobsters to Japan, and the fishermen haul their catch in each day at the dock. I believe the season is July 1st through February 28th (though I'm not 100% sure). If you're visiting in season, you can buy fresh lobster, either to cook (from the local collective adjacent to the police station) or at a restaurant. Expect the lobster to run ~350pesos/kg to cook or around 300 pesos for a serving at a local restaurant. They are delicious, but you will find them different in taste from lobster you might get in the U.S. or Canada (for one thing, the lobsters here have no claws).
Serenidad Shardon is just south of the village. To reach it, drive down the main road through Punta Allen until it ends at a basketball court. Hang a left and then an immediate right at the police station. Drive down an additional 1/10 mile until this road as well dead-ends in a t-intersection. Turn left there onto a sand road, and follow it slowly around to the right for about 100 feet. And you're there! Once in your room, you'll notice a laminated map which has useful instructions for reaching Serenidad Shardon (lamentably these are not included on the website).
The view from the rooms that are beachside is phenomenal, and there are wonderful little sitting areas with chairs, loungers, and hammocks scattered around the property.
Though the Si'an Kaan Biosphere Reserve is a wonderful place to go, I wouldn't choose to stay in Serenidad Shardon again. Even in January, there are tons of mosquitos in the area. It was virtually impossible to sit outside once the sun started going down due to the mosquitos - even after we'd applied the toxic DEET that we brought with us to our skin and clothes, we ended up with mosquito bites all over. Our lodgings were cute and attractive, but it was impossible to keep the mosquitos out. There were large holes in the mosquito screens near the ceiling in several rooms, and the mosquito nettings draping the beds also had holes in them. I was able to mend them using my sewing kit, but we had to sleep under them in order to avoid being eaten alive. With no fans in the sleeping areas, and the mosquito nets blocking the breezes, we found sleep difficult even in the middle of the winter. I was profoundly grateful that we weren't visiting in the summer. The bathrooms are clean but pretty dismal with a light trickle of water in the shower. I wouldn't have wanted to go in without my shower sandals on, and had to wait about a minute to get enough water on my head to rinse the shampoo out. If we hadn't prepaid, we would have left the day after we arrived.
The kitchen areas were amply supplied and the perked coffee was quite decent. The rooms were cleaned every day by the efficient local staff (which was helpful, they cleaned away the cockroach bodies that we found). The beds and sheets were clean and the rooms were decorated simply but in a fun way. Access to the ultra-slow wireless satellite internet is provided at a nominal fee and is available during the hours when the electricity is on.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC