i love sea days! it gives you a chance to relax, roam the ship, take part in activites if you wish.
Lots of folks stay on board when the ship is in port. No crowds by the pool, good discounts at the spa.
We usually do go ashore at least for a little while, just to explore and maybe replenish some supplies, if we are visiting a port that we have already visited in the past we may stay on board.