I would recommend reserving chairs/umbrella at either Yellow Beach (my preference) or Karibuni at least a day or two in advance if you will be in SXM during peak Dec-Mar timing. The beach at Pinel is normally quite calm since it is so protected and it is an easy flat walk into the water. I always find the "swimming" there to be more wading because it is fairly shallow. There is a short walk on Pinel to the north side beach (which is sometimes all chunks of coral instead of sand) that is worth doing. On that walk, you get a good view of SXM from a different perspective. Note instead of taking the ferry, you can also rent kayaks from Caribbean Paddling (located on the ferry dock) and it's an easy paddle over. If you want to do the kayak thing, definitely reserve those in advance too.