On the other days, I wouldn't be afraid to book via a third party.

One our first cruise I booked through the ship. But I haven't book through the ship against since.

There are some reputable sites, like viator, that will have a bunch of excursion activities to choose from. Most of them have the timing of the excursion and you can pick the window you want. They make their money off of people on cruises, so they will lose out on customers if they get a reputation for not getting people back in time to get on the ship. Just pick something that won't take all day, and you limit any risk. I'll bet you can find something swimming with pigs through a third party pretty easily if that's a thing there.