You need a Y pipe if you are going to use the raw water intake for the return. That will help low RPM heater performance a little, but it's not miraculous compared to returning to the engine water pump.
For reference, here's where PCM says to tap for the heater (remember that indmar, PCM, and everybody else uses essentially the same long blocks):
Note that the plug in your picture is the one on the top of the pump, while PCM says use the one on the side. It's a fielder's choice, either will work.