If you're wanting to camp in a tent then you have to pay the fee for the area you choose. To just swim you have to have a parking permit which you can get at the ranger station or when you renew your tags... That is for the entry/parking only though.
I would suggest getting the Park pass. It's good for all state parks
It's $5 a day or $25 for the year, there is also Cabins & a shower house.
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