Luquillo beach in Fajardo is a great spot, about 2 hours east of San Juan. Please watch your selves and your belongings. Do not leave valuables in the car.
However, if time is short, go to the Condado area where the major hotels are and walk in like you belong. You will find excellent beaches and pools with good security. This is a short taxi ride from the pier, (under 20 minutes, traffic permiting.)
Mind you that this is the Atlantic side of the island and not the aquamarine calm waters of the Caribbean. Have a wonderful cruise and please come back to tell us about it!