Advice please

Bottom you say? How long were the exposures? A small light leaking in from prism side usually does make a difference on higher shutter speeds, but if this is on tripod longer exposure I could see the light being an issue, in fact they have the cap for the eye piece on the camera strap to add a extra safety gap in those cases.
The problem i see is that is very noticeable, that one on the left shows some shadows that make me wonder if the mirror is not swinging up all the way and possibly jammed at an angle? Idk I have not dealt with such an issue myself, but that is what it kinda looks like. Have you taken the lens off and examined it? Maybe put it in sensor clean mode so you can examine the mirror raised and see if things look ok. In any even this is likely going to have to go in for repair. You can also try other lenses but seems to me the issue is in the camera. Unless you have a lens on that is somehow in the way of the mirror? I assume your using multiple lenses with EF mount and seeing this issue.

 

I guess i skimmed over the water issue. It did look like some weird light issue so I focused on maybe a mirror box issue.
Now that I took a sec and remembered the water was mention, how wet did it get? While not a good thing, or a test i would try, I would expect it to fair better if it got splashed (the 90d has weather resistant sealing). That said if you git it soaked water could have got in and damaged the sensor.

Another concern if it gets water inside, is corrosion, the circuit boards can produce issues if the inside has corrosion build up.