I would definitely book your lodging that time of year - many of the coolest and best value places will already be booked. I'd choose between Caulker and Ambergris - maybe based on availability of lodging in your price range - rather than try to split between the 2.
For what it's worth, I always book my lodging in advance. I like the idea of winging it and keeping more flexibility but I love the research phase of a trip and typically fall in love with specific options and don't want to miss out on them. I also know myself and my travel style well and make good guesses about the amount of time I'm apt to want in an area. Since I return to CA every year, anything I miss 1 trip is just another excuse to return. If I have groups in tow, having reservations is especially important because it's harder to fit a large group in and also much less pleasant to go from place to place looking for space with a group waiting.
I use Trip Advisor to research my options so there's basically no risk of showing up and finding it is "next to a garbage dump or late night disco" as Roberto suggests, though without Trip Advisor reviews that could happen often in CA. In about a dozen trips to the region I've loved every single place we've stayed - no regrets.
Have a great trip, whatever you decide!