There is an app to check water quality. In general it's fine to swim in, but at high temperatures sometimes algae form. That's common in lots of these kinds of lakes in the Netherlands.
Inaccessible lake homes being allowed to be used as a primary residence will not fix it.
why not?
Given this is the Netherlands and this appears to be a wealthy area, I'm quite sure they don't. Guessing they either have septic tanks or holding tanks that need to be pumped regularly.
----
There is a running joke (hundred+ years now) that the famous "Ruby Falls" on Lookout Mountain is in fact better named "Poopy Falls" – since the primary water source are the few hundred septic drainlines higher-up the mountain's-top.
<https://en.wikipedia.org/wiki/Ruby_Falls>
----
If you have any open cuts/sores, do NOT swim in any outdoor water.
Typically you use them for the weekend. My mom for example lives in a busy city and has a small garden, she uses the holiday home as a weekend getaway and a place to keep the sailing boat.
The Netherlands are small and this area is very central, about 20 minutes drive from Amsterdam or Utrecht. So most owners use it in the same way.