Good luck mate, the sooner you get the "corrosive" sewage water off the bikes the better, before it dries preferably
Nasty stuff that sewage water, rots stuff in no time as it contains all those horrid things people put down their toilets
You wouldn't dream of washing your bike with any of it would you
I know you have a lot to cope with other than the bikes, so I wouldn't want you to think that I am trying to tell you how to "prioritize"
Just trying to warn you mate, not to leave it too long if you can help it
If its only on the lower parts of the bikes a "jet wash" on low pressure should shift most of it
I do hope that the rain abates and the water level continues to subside
My thoughts are with you at this difficult time