My favorite beaches in P.V. are Yelapa, Piedra Blanca, and Sayulita. they are a distance by bus, cab, and or water taxi.
Los Muertos beach is great if you want to be near downtown for shopping, dining, etc. Also there is a very nice beach right at the Krystal Hotel, which is directly next to where the ship will dock. If you ask the Krystal, they will let you use their pools, restaurants, etc. The fee should be nominal and in some cases it is free.
We're going to be on that cruise as well and we can't wait. P.V. is one of our favorite places in the world. It's a great city with a lot to do. We have friends who live there and will be visiting them for most of the day in P.V.
i simply cannot take anymore of chucks yalupa suggestions, sorry. if you know P.V, this beach is simply not in the league of the marietas, majuitas or las calitas (may be spelled wrong). the sand , water clarity and overall scene is much more inferior than the beaches i've mentioned--all only reachable by water taxi. yalupa is a hippie scene and while it is quite nice, the other beaches are more worth the ride out.