We go the first week or two of Dec and it's typically nice weather. We wear jackets in the morning, by 9-10 am shed down to long sleeves and jeans, after dinner we start layering and usually wear a jacket again. I like to pack some warmer clothes for the kids, but I usually just use a warm jacket at night (although sometimes I layer a sweater under the jacket). We've always swam, but it's chilly lol kids.... this will be our fourth year and we've only had rain on our week-9 day trips maybe 3-4 days total in all the years including an "El Nino" year, but pack some umbrellas, ponchos and changes of clothes/shoes just in case (stroller cover if you need it). You'd probably be ok in a short sleeve t-shirt but rarely even walking a lot did we regret wearing long sleeves all day and a lot of our photos were wearing jackets.