Have a look at www.bolsters.com
for one idea -- they make seat units that have three positions which they descibe as:
1. Superior sitting comfort - equivalent to luxury cars
2. Lean into bolster - relaxed comfort with side support
3. Stand up bolster - protection equal to offshore racing keyhole bolsters.
There are also various designs of bucket seats as used in racing RIBs, but I'm not convinced that they would do much to prevent back injuries. You could try talking to one of the RIB manufacturers who specialise in race boats, such as South Coast RIBs or Revenger.
Seats with suspension/shock absorbers appear every now and again, but I don't know of one which has been succesfully marketed (they seem to be one-offs mainly). The biggest problem with these seats is that they are liable to break, and when they do break they become useless at best, and quite possibly dangerous.
In my opinion, the best way to avoid back injuries on RIBs is to stand up!