Hi, i grow up in close to that lake and as i remember you can access on two ways from the Havel (River).
1.) When you'r on the Havel to the right of the gate is a stair up to the street. Cross the bridge to the left side of the gate and go down that little pier. That pierplattform is about 1m over waterlevel heiligensee and easy entrance with a SUP.

2.) Go the Havel south till "Sandhauserbuch" (Beach) and walk back to the "Strandbadheiligensee" (public bathing beach) -> But that is a 10min walk.

See ya at the water!