First, I want to encourage you to speak up if you do feel that something is wrong. DS has been officially diagnosed as Gross Motor Delayed, and is in PT once a week now. (We were in PT early on for his torticollis, but have had to up the frequency in addition to at home excercises. )
He is also in the top percentiles in all measurements. So much so, that his 6 mth head measurement landed him a diagnosis of macrocephaly and an emergency ultrasound to ensure there is no hydrocephaly to blame for this (we got the all clear, thankfully!!).
While "most" times, you are right, babies develop at different speeds, and do different things at different ages, I think a lot of times, we are told to just wait and see. Or at least I was, which is why I think its important to speak up. Hopefully, it's just a stage that passes, but I would familiarize yourself with what things that baby "should" be doing be 9 months for sure, so that you can go to his check up prepared.
I am also very frustrated with the bath situation, it is amplified by the fact that his head turns severely to one side, so it's impossible to get enough water in the tub, without having at least one ear under water.
Hoping that your LO moves fast soon, I understand your frustration! :)