Great to see posts like this that share a project for others to see.
Thanks for posting.
BUT, it looks like you put the 'Y' pipe on the wrong side of the raw water pump. It should be placed on the intake side where the pressure will be low so that extra water flows through the heater system. With it on the pressure side, you will get little flow and may even flow cold water through the heater instead of the hot.
Also, the stock location for the heater in a 190 is mounted on the underside the bow. There is a section on the starboard side with a good mounting plate (unless they only glass that as an option - not sure on that one). It is somewhat nice to have the box and all that hose off the floor, but yours looks nice where it is.
great job in general.