I think 'playas' refers to the foreshore, the area between the high tide and low tide marks, between the seashore and the start of the built -up area on land, often referred to in Spanish as the 'litoral'. Hence the insurance offers coverage for the buoy-marked areas which are monitored and are designated areas, as well as the foreshore itself, which gets discovered again(comes to life again) only in the summer months. I think 'descubierto' means 'discovered' and not 'unsheltered' here.

Almost certainly in the context of an insurance policy, in relation to where the boat is being kept.

Winterizing Tips: Storage Ashore
In some parts of the country, where winter means several months of bitterly cold weather, storing boats ashore is the norm. In warmer climates, however, ice and snow may occur infrequently, and the choice between storage ashore and storage in the water is open to discussion.

Storage in the water means you might get a jump on the boating season next spring. On the other hand, boats stored ashore (on high ground) won't sink. If you have a choice, storage ashore is a safer bet.

Storage ashore may also be less expensive over the life of a boat, since a hull surrounded by air for several months each winter is less likely to develop blisters than a hull that remains in the water. These blisters, the fiberglass equivalent of rot, occur on many boats when water soaks into the laminate below the waterline.

Here there is a photograph:

Playa Descubierta - ( GUARDERIANEPTUNO )
www.guarderia-neptuno.com.ar/playa.htm - Translate this page
Playa Descubierta. Servicio de Playa Descubierta: Contamos con seguras camas para embarcaciones de hasta 8 metros de eslora, las mismas pueden ser ...