I have had pretty good results using a pressure washer on inflatables. If you can leave the boat out in the sun for a day or so before, it helps.
On a side note, I have found out that some inflatables dont do so well in the sun. Zodiac
in particular. Its a good boat, but UV eats it. Keep Zodiacs in northern waters, or get a cover for them. And heres another tip, learned the hard way: dont use Armor All or any silicon-based cleaner/enhancer on zodiacs. Why you ask? well, cause once the silicon gets on the material, it stays on it. Then when you need to patch the boat the new glue/patch doesnt stick well to any material that has been in contact with silicon.