Swimming places in Lake Siljan and central Sweden

I haven’t ever gone swimming in Lake Siljan, but here is an article with a list of the 7 ”best” swimming places in Dalarna. It is in Swedish, but they have urls for each location.
https://www.expressen.se/allt-om-resor/dalarnas-basta-bad/

Alternatively, this site
https://www.badkartan.se
Lets you search for bathing places around Sweden. Just enter a place name in the search and you will see a map of what is nearby.

I’d ask a local for a recommendation when you get there