In a heavy saucepan, bring strawberries, raspberries, and sugar to a simmer. Mix together water and cornstarch and stir into berry mixture. Bring to a boil. Boil, whisking constantly for 2 to 3 minutes until thick. Remove from heat. Cool completely. This will make about 1 cup of filling.
Whip cream using an electric mixer and whisk attachment until stiff peaks form. Refrigerate. (no need to add sweetener to this.)
Mix vanilla wafers, butter, and sugar together. Press into a greased 9-inch springform pan. Refrigerate until set.
In a large mixing bowl using an electric mixer, combine cream cheese and sweetened condensed milk until completely combined and smooth.
Fold whipped cream into cream cheese mixture.
Pour into springform pan.
Drop teaspoons of berry mixer on top of cheesecake.
Using a long skewer, swirl berry mixer into cheesecake using circler patterns.
Freeze 8 hours before cutting.