I don't know that the sale carousel ever stops.
If you see a 30% discount on the drink package I'd snag it as I doubt it gets any better than that.
I think excursions run 10-20% off, usually 10% when there's a sale. If you're doing a tour that's exclusive to the cruise line, then that's fine, but I don't think it's enough to offset the price savings from going direct through a 3rd party.
With that said, peace of mind that the ship won't leave without you: priceless. If you don't have that comfort level about booking with the 3rd party companies, go through the ship. For example, if I'm doing anything that involves the phrase "take the ferry to Playa del Carmen", I'm booking through the ship. Otherwise, my comfort level is pretty high with 3rd party companies.