We're going in July and I insisted that everyone going (there are 8 of us, and 7 are participating in the excursions) make up their minds by early January, so we could assure a) availability and 2) an affordable price. We lost my last grandparent in Dec 2022 (my grandmother, whom we all called by her first name, Kate) and my dad received some money from her estate, some of which he shared with my siblings and I, so the money was there, I just wanted to make sure that we didn't spend it all in one place. We kinda did, since all of the excursion portion is being spent in Cozumel.
In summary, the sooner you know what you want, the better it is to book it right away.