I think I posted this somewhere else before but since many of you are headed there soon, here it is again: You are tendered to both places and both are beautiful beach / swim / snorkel relaxing areas.Grand Cayman has Clam Shells and Beach Chairs/Umbrellas that are free if they have a green umbrella. Locals are selling chairs but there are ample free setups provided, (if you know what to look for). The Margaritaville Pool is also free and has a wide range of areas to relax. Shops/bars take cc/cash but not cruise cards. Princess Cay is owned by Carnival so all beach chairs are free and the BBQ and fruit offerings are free as well. You will use your cruise card to pay for anything on Princess Cay (except a few local stands that take cash) so your drink packages apply. Even if they announce that towels are available onshore, take some from the ship as I have found that to be false twice on Princess Cay.
You can rent just about anything on either island (snorkels, floats, etc.) but this is where I take my own. My floating chaise that springs open is light to carry and perfect for both places (sea is calm in the area between beach and where ship is anchored)
The returning tenders have long lines close to departure, so go back to the ship a little early if you don't like to wait in line.