Balneario Monserrate is also commonly known and marked as Luquillo Beach and is a few hundred yards east of the Kioscos. The area behind the Kioscos is NOT Luquillo Beach and is usually a bit trashy and unfortunately gets confused with Luquillo Beach. This is why you sometimes see mixed reviews.
Balneario Monserrate is much calmer that the beach at Rio Mar, a beautiful setting but a bit boring if you like playing in a little surf. It is a very easy drive and the parking is only about $4.25 for the day.
It would be a hefty walk from Rio Mar via the highways which don't have sidewalks. I don't want to malign PR drivers, but let's just say you don't want to join the plentiful road kill. There is a very steep hill both up and down which would be a killer on the way home.
If you are prepared to do just a little wading, you can make the trip along the beach which is considerably shorter, probably 1/5 the distance. You will have to ford a small river, about 1-3' deep depending on the tide and wade around a small rocky outcropping on the beach. Because you will have to judge these water hazards, I would only do it during the day.
Kiosks are open mostly at night but usually not terribly late. Most not much past 10 but there are a few open later. Monday / Tuesday the bulk close after a busy weekend.