well, first off it is soft plastic, so the trick wil be to find summink that whips the crap off like in the adverts, not with massive scrubbing. Else next time it's be really reallly hard to clean.
Now, I'm suddenly a domestic star this evening cos wife asked how to clean the kitchen table. Firsly, I sugested sacking the cleaner and and getting someone who didn't think that cleaning involved spraying polish over things. Then, nipped into garage and got some petrol, and all fabulous polish and crap removing amazement ensued. Ten minutes work for five zillion brownie points.
Therefore, try soaking a rag with a bit of petrol, and I bet it'll work. Note that the petrol evaporates dead quick BUT it is much much cheaper than "inflatable boat cleaner" at only four quid for a whole gallon. Also, the boat will be fine against petrol cos otherwise everyone would have busted dinghies with slopping petrol around. The wash off the petroly bit with washup liquid.
The best thing I have used is Truck And Curtain Clean - used by lorry drivers to clean the side curtains on lorries. You can buy it in motor factors. Takes everything off, including skin so wear gloves!
Try t cut...discovered this by accident but it will bring the dingy up like new, it even worked wonders on a narwhal dingy whose tubes had gone 'sticky'