If the motorboat maintains constant speed both up and down stream, relative to the speed of the water, then the time taken to travel in each direction would be 45 minutes. This information would in no way help us to find the speed of the water current.

The question correctly put would be that the speed of the boat is 15 miles per hour and the time taken for up and down journey is 1.5 hrs. In this case we can determine the speed of the water current. If this is x miles per hour, then speed of the boat upstream would be 15-x miles hour and speed down stream would be 15+x miles per hour.