At a lot of our parcs, for hygiene reasons, it is compulsory for all swimmers to wear 'conventional' swimwear - in other words, actual swimming trunks are to be worn. Bermuda, or long-style shorts are not allowed in the pools.
Swimwear regulations are particularly common on parcs in France and some parcs may also require swimming caps to be worn.
This is to ensure the pools remain as clean as they can be, preventing the filters from getting clogged up with any dirt or grime that may be transferred from shorts that have been worn while engaging in other day-to-day activities outside the pool.
Some parcs also have specific regulations against wearing t-shirts in the pools.
Please check the individual parc pages for any specific rules regarding swimwear at the pool. Once on your parc page, open the 'Pools' section and look out for 'Restrictions'.