On recent cruises to the Carib, we found that many cruisers had the same plan to stay on board.("been there, done that")
Guess it is flexible..and depends. We usually pick one port day to stay on board..esp if a tender port
On Ascent, San Juan was a day where it was fairly crowded at the pools all day ...we stayed on board but did not have the ship to ourselves at all..Outdoor Pool and Solarium were fully packed.
But on Beyond, when we stayed on board (not sure which port) , we lucked out and enjoyed sparsely populated Retreat pool and lounge areas! It was great, til folks returned from port, but we were ready to go anyhow!