Answer:
Húmedas, tranquilas y claras.
Step-by-step explanation:
The noun "beach" in Spanish is translated as "playa". When you add the suffix -es to "beach" in English you get the plural noun "beaches".
In the case of Spanish when you add the suffix -s to the noun "playa" you get a plural noun "playas".
As you can see adjectives do not need to be pluralized:
I would describe Puerto Rico's beaches as wet, quiet and clear.
However, in Spanish the noun must agree in number and gender with the noun:
Yo describiría las playas de Puerto Rico como húmedas, tranquilas y claras.