Ashore time will be listed in your app/plans, and it varies based on itinerary. Note too that it's an estimate, and can be a little earlier or later depending on docking, CMs going ashore, etc. As already noted, they'll know if the ship is late docking and will adjust your excursion accordingly.
I've parasailed on CC twice and loved it! I'd recommend doing the earlier slot-it'll be pretty much right after all ashore, so you can do do that first then you have the rest of the day to do the beach/explore the island, etc. If you do the afternoon slot then it's smack in the middle of the day (and you'll have to 'come back' from wherever you're at).