I would take a boat from Port. to City Walk, and from there catch the boat to RPR. There is a shuttle that runs between the 3 hotels and US. I know it drops you off at the end of RPR Tower 3 where Tchoup Chop is. Don't know where you catch it at Port. though. Pretty sure it's much too far to enjoy walking the whole way between the two though.
The bus would be the most direct route. I think you get the bus in front of the PB. I agree though, the walk from City Walk to RPR is quite nice, so you might just want to take the water taxi to City Walk and then walk to RPR.