What I really need it a "magic" cleaner. Or someone else to do the cleaning......
Is there a
fabric covering the rubber hypalon pontoons? If not, then the maker used rubber compounding agents, like carbon black. If you have chemically altered the rubber or compound agents, there is no such thing as a "cleaner". You might try an automotive polishing compound (fine grit) to buff out the damage.
Picture 1 shows streaking, like something dripped down the sides and "cleaned" it. So if your water line of you floating dinghy is at about the top of the stained area, then the cleaning (streaking) took place
after you took the dinghy out of water. Get my drift?
I like the challenge, chemically speaking.

_______
I follow
@Rich Stidger for several reasons, one is he and I have the same year and model Volvo Penta engine. I am now working on making my engine look as clean as his, per his advice. If I remember, Rich is the Original Purchaser of his boat. Thus he is "keeping up" and not "catching up" like me.
But there are several level of cleaning...
1) Quick hide stuff, unexpected guests.
2) Sweep the dust under the rug
3) Sanitary smells and mold removal.
4) Detail cleaning
5) Prideful maintenance remembering how she should look.
6) I will get to it later, lets go sailing!
Rich you could do level 2) and buy a new dingy cover.

Jim...
PS: Remember as we age, we all get a few wrinkles.
