as andy m. says, the water bath should go far towards helping, but another couple of points are worth bearing in mind. the water protects the sides from overcooking because water never gets hotter than 212 degrees (100celsius). if it was still too soft in the center after an hour without the waterbath, it'll take even longer with it. you may need to tack on another 10-15 minutes or so, especially if by "very creamy" you mean too soft. another tactic would be to use a larger diameter pan to decrease the height of the cake.
the recipe looks very standard. if you are still running into problems after the second or third cake, you could try: increasing the flour a teaspoon or so, use another egg, decrease the sugar, decrease the milk.
substituting cream for the milk will be fine. personally, i don't care for the flavor of condensed milk, so i wouldn't use it. if you like the flavor, you can use it but will probably want to decrease the sugar a bit.
what kind of pan are you using? i take it that you aren't using a spring-form pan.