Water ballast is quite common and dates back 200 years or more. Avon never had an patent on it and it was originally used on the first 4.5m way back in the late 60s.
There is some confusion as to who developed the first RIB, wikipedia puts down to Flatacraft but this was an abortion and the first true RIB as we describe them today came from the Isle of Wight, the designers name was George Marvin, but it was this boat that Avon used as the first and subsequent Sea Riders.
Originaly the Sea Rider was marketed to the leisure market with little success, as it was neither one thing or another. It couldn't be rolled up for easy transport as an inflatable, it was very unstable as dive platform and it was too small to have as a trailer boat, with all the messing around and towing speed restrictions and inconvenience.
Attention was focused on the dive boat market and they were made light enough to be able to put on a roof rack of a car, this also gave them quite a good performance on small engines (20 - 30hp) which were easily carried in the boot of the car.
To overcome the stability problem they decided to use a flooding keel so that when at rest the boat would sink into the water and rest on the collars. This was a good solution and the boat started to find a market. The flooding keel was used on the 4.5m and the 5.4m Sea Riders.
The real breakthroughs came from when Avon got the Royal Navy to put them on board ships and more importantly the North Sea oil and gas industry. The Navy used them as run arounds until the introduction of the Halamtic 22 when these boats were taken more seriously, and the oil industry pushed things much further by developing them into serious all weather rescue boats, and you have to look at companies such as Delta in the UK and Norsafe in Norway and to a limted degree the RNLI in the UK as the pioneers of the go anywhere anytime RIB