We did it about 3 years ago and the catamaran was large, with shade. But best to try to be one of the first on, if you want seats in the shad.
"My wife and I first did this tour through Barefoot Tours when we were in St. Lucia for a week and then I took my mom on the one through RCCL when we were on the AOS. The one through Barefoot was so much better! First, when you use the ship, there was a larger group, 85-90 or so as opposed to 40 or so on our first tour. The catamaran RCCL used didn't actually sail, but ran it's motor the whole time, never putting up the sails! The Barefoot Catamaran sailed down and only used its motor so we didn't have to track back. The only other real difference was that on the Barefoot trip we went to a restaurant for a buffet and it was a gorgeous open air restaurant right near the botanical gardens and there was a great view and waterfalls, etc. with very good food. On the RCCL trip they gave us lunch on the boat and the food was not good at all. The crew on the boat were nicer on Barefoot as was the Catamaran itself. The one Barefoot used had the roped netting up front where you could lie suspended over the water. The Catamaran RCCL used just had folding chairs and the deck was all metal like a ferry boat. I think Barefoot charges $90 per person, but it is well worth the extra bucks. Both boats leave from the same place. I highly recommend you switch and do it on our own. The only advantage RCCL has is the guarantee of not missing the ship, but I have never heard of one of these day sails missing the ship, so don't worry about that."
Yes there is shade,no need to worry and its a fantastic trip...enjo.