The dream indicates that you are creating a new mental attitude, one reflecting new opinions and beliefs with which you could feel "at home" or comfortable. (building a new house). This is being done using thinking pertaining to your ex-partner. However, you do not seem very clear about the nature of the relationship you have with him.
Your mind, using the ex's thinking, would have you establish the principle's you stand on to reflect spiritual thinking (floors of water). The flowing water indicates spiritual thinking from a living source - from God Himself. Yet here this kind of thinking could take you too far too fast. (water picks up momentum) Note that fast moving water is like a river, suggesting a spiritual course of action that might take you too far too fast.
The young nephews represent an innocent, more unintelligent and naive aspect of your mind and its thinking. Here such thinking cannot safely use a spiritual course of action. You might want to hold back this naive thinking and even wonder if the spiritual thinking being considered is a good idea here. As the nephews are relatives, the innocent thinking they represent can be regarded as being "relative", that is, it depends upon the conditions in which it is used.