I found some cushioning rubber tiles... 12" square... that do a great job. Initially, I wanted to use them to protect the deck from dive tanks etc., but found that they really make standing in rough seas a lot easier. They also have a lot more grip in the wet.
The only complaint I have is that they have a tendancy to slide aft, so get a good heavy weight of mat or tile. Also, I ended up tieing them together with zip-ties because the mechanism they use to lock together wasn't adequete for the pounding they get in a RIB.
This:
http://www.usmatandrubber.com/sport.htm is the sort of idea, but mine is an open grid style which allows the water to drain through, and also allows me to use the zip-ties... I suspect if you check with your local chandler, they should have access to something suitable.