Don't know much about what size would fit on a 175 but I have a 90/65 on a PT 185. Never GPS'd it but I would guess it runs in the mid 30's. It will run in some very skinny water, and therein lies the problem. I can get into some very skinny water but once it comes off of plane I can sometimes get stuck or hit bottom.
The mod-v hull allows you to run in some chop on lakes but if it gets too rough the motor will cavitate as pmgoff alluded to. But for shallow river running it does fairly good. Maybe not as good as some flatter bottomed boats but pretty decently.