We will be on Maui in January. How far in advance should I book things like snorkeling tours and whale watches?
The experts on here will know better, but my rule is to book any tour as soon as I know my date. I always check the cancellation policy but to me it's better to book and cancel if needed rather than to find the date and time I want is already booked because I waited too long. I do this for tours and some restaurants.
Schedule it early in your trip so you can change it to a later date if the weather is bad on your original date.
January is not as busy as other times. Even in busy times I maybe do a couple weeks in advance... but others do months in advance. Just depends on what you are comfortable with.