SR's were pretty well designed (especially so when they were current compared to lots of the other Crap they competed with) they though didn't get everything right...the Consuls and seating have always been on the very low side for me at 6' and at speed in the rougher stuff ...while the Hull performed well it was always a struggle just to hang on!
If you do find "inflatable seats" that actually WORK (which I must say doubt) I'm sure plenty on here would like to see them..
In the interim..my advice is the simple tried and tested method used by all... Just use you're knees to grip/flex and mitigate impacts!
If this doesn't help in rougher conditions you have two further choices...
Either ....Slow down
...(and remember it's always worse for passengers!)
Or..Buy a Bigger/better equipped RIB