Aloha,
We have been in Kauai in December and January for multiple weeks for many years now. We always bring our snorkel gear. Most years we get to use it. Some years we don't. Reasons that prevent snorkeling include high waves, but impaired visibility is the primary culprit. For instance, on Kauai, Anini Beach is so shallow for hundreds of yards out from shore that the water is calm enough to go into even when 30 foot waves can be seen in the distance. However, the visibility can be so bad that it is not worth snorkeling.
Last year, the rains were so intense and the erosion / run off so thick that it was pointless to go snorkeling most of the time we were there unless all you wanted was the exercise. During our three week stay we were able to get one day in at Anini where my adult daughter and I seem to have a competition of who can encounter more turtles (she usually wins), one short day in front of Lawaii Beach Resort (marginally ok visibility but got out when the shark arrived), and one short immersion in a spot not recommended without all sorts of preparation: Ahukini Landing. In all cases, visibility was impaired.
A shorty is highly recommended but I did make do with a neoprene vest for several years.
Jack