It's a term used by some printing companies or packaging manufacturers for a foam or rubber insert used in packaging with spaces cut out for items. I think that's what they must be referring to here. I think it's usually called an insert in English.

"Diseño de Packaging para Telefónica Seneca Express
[...]
Un estuche de 22×16,5×2,5cm. forrado con lámina impresa a 4/0 tintas con lleva cierre de imán en la solapa y piscina de foam para el alojamiento del pendrive."
http://www.artesgraficasg3.es/diseno-de-packaging-para-telef...

You can see it in a video on this page. It's a piece of foam in the product package with a hole in it where the USB stick fits.

Another example; they're talking here about magnets as closures for packaging:

"Por último la caja negra, lleva una solapa que cae sobre la vertical delantera, donde oculta dos imanes de 10x2mm. La caja lleva en su interior una piscina de caucho EVA que aloja un catálogo y una memoria USB."
http://www.artesgraficasg3.es/los-imanes-de-la-industria-gra...

Just to show it's not just one company, there's also this, though it's less explanatory:

"Compuesto de tapa+bandeja impreso a 4/0 tintas, sobre papel novatech matt de 150 grs, plastificado mate 1/c y piscina +3."
http://www.agsmartesgraficas.es/2010/

And one more:
"Estuche en cartón compacto y piscina de caucho negro con cierre de imán o piscina con collarín para cierre de tapa o piscina en folding con velcro en el cierre."
http://krixar.com/estuches/