Answer
The ocean in Uruguay is quite cold and the actual swimming season is limited. Swimming is usually possible for a couple of months a year, and a broader window for going in the water runs from mid-November to the end of March.
Even during this period, conditions are not always the same. On the ocean beaches in and near Punta del Este, there may be wind, waves and cool water, so how it feels depends not only on the month but also on the specific day.
If a beach holiday is the purpose of the trip, it is best not to bring expectations of a tropical sea to Uruguay. The coast is appealing for walks, views, the port and its resort atmosphere, but the season for extended warm-water swimming is short. A windbreaker and a backup plan for a cool day will come in handy.