I would suggest a mild abrasive WITHOUT a beaching agent, such as "Comet" no bueno.
Maybe a diluted mix of baking soda and water.
I used to raft a fair amount and maintenance was paramount.
I would resort to Powdering my raft before putting away at the end of the season.
To keep your raft, cataraft, inflatable kayak and SUP board looking new, use Inflatable Boat Cleaner. Easily removes ugly dirt, stains and frame marks.