Swimming in the pond in Vallisaari or the cliffs is prohibited because of safety issues. You can go swimming in the sandy cove, located in eastern side of Kuninkaansaari island. Note that this cove is not officially designated as a beach so it is unattended and in natural state.
Please note while swinning, that Santahamina Military Area is on opposite shore to sandy cove. As military area, there is 100meters restraining order to Santahamina shore. One can't land to sandy cove with motorboats or other motorized vehicles such as jet skis.
On the road across the sandy cove are toilets.