We found a bunch of these balls across "Rio del Plata" in Montevideo or in the coast of Uruguay (Punta del Este).
There is a liquid in the ball. Also, it has a soft skin.
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It might be a large specimen of Valonia ventricosa, also called "bubble algae" or "sailor's eyeball".
If they haven't exploded yet, in addition to liquid, you will find some embryos inside.
They are very popular among Uruguayan coasts, that lead to wrongly associating them with shark eggs or turtle eggs.
Commonly, they are known as huevos de caracol negro (EN: black sea snail), voluta negra (because of its family: Volutidae) or ovicápsulas con embriones de caracoles (ovicápsula: from Latin: ovum: egg and capsŭla.
Technically speaking: Adelomelon brasiliana.
This yellowish capsule, it's filled with food supplements for the embryos, it is located on the seabed and has an oval form, being this a unique feature between sea snails worldwide. After strong winds, many of them remain on the coast, with very low chance of surviving because of the depth where this specie raises. After approximately 2 months, eggs hatch in between 10 and 30 embryos.