It all depends upon what you want to do. If you want to rent waverunners, or go snorkeling, probably a good idea to reserve in advance. All we did was just plunk ourselves on a chaise, went swimming, walked around, had a cocktail, its a beach party day!
You do not need to reserve anything if you just want to hang out, swim and enjoy the sun. If you want to rent items then might be best to reserve them.
We personally hate the cruise lines islands and find them boring so we go ashore for about an hour then back to the ship to enjoy the day. But if you enjoy the beach and swimming then by all means go and enjoy.
The excursion around the island is boring at best. Save you money & relax for the day in a chair without spending any money.
btw- There are no free seats as you leave the tenders going to the right- only clam shells which come at a cost. All the free chairs are on the left side, so get there early if you want shade.
We were in Princess Cays last week on Grand Princess. We reserved the clamshells in advance, but really didn't need to. There were plenty available, as most people don't want to spend $29 per couple, and would prefer the free lounge chairs.
When you exit the tender, and walk across the dock, the clamshells will be on the right, and the free loungers will take up a HUMONGOUS area on the left. The bummer with the set-up is if you are in a group, and some don't want clamshells but some do. There is no way to drag the free loungers over to the clamshells so you can hang out with family/friends. My husband wanted the guaranteed shade of the clamshell, while I like mostly sun. We had friends who were also split like this, so our group of 7 split up and didn't enjoy the day together as a result.
There ARE several large structures that provide some shade in the area on the left, but the loungers are packed under them like sardines, and it's too hard to move them because they're all scrunched together. As the day goes on, obviously the sun/shade will move, so if you want guaranteed shade, grab a clamshell.
Waverunners and snorkel equipment were plentiful, and I so no need to rent those in advance.