I've owned 4 boats in my life, one with a 150 hp motor, one with a 115, one was a 60 hp and the one I have now has a 2006 90 hp mercury, it the same engine as the 90 yamaha just has a different name logo on it. and never had one with a water pressure gauge, I've replaced the water pump [ impeller ] on mine twice, I watch for the little pee stream when I start it and a good strong pee stream shows the water pump is working good.

ps, I made a video of replacing mine on a 90 hp Mercury and put it on youtube, it isn't a hard thing to do, watch the video and do it your self and save a buck or two.

" how to replace water pump 90 hp Mercury outboard "