Use the muriatic acid, but be careful of the fumes and don't splash it on yourself.
Pour some in a bucket and use a brush to apply it.
Try not to breathe the vapors.
The thieves at west marine and mary kate should be arrested !
I bought some wheel cleaner for $6 and tried it on some aluminum wheels on my volvo that were neglected by the previous owner. I might as well have pissed on the rims ! I mixed up some Muriatic, and cleaned up hardened up brake dust like nobody's buisiness.
I think they like selling water at top dollar, and because of the ignorance out there they have to make the product weak to protect themselves from liability. BBB

on/off or muratic acid DOES eat into the gel and slowly damages it. Each time you use acid, you stain will return faster and faster and will be worse.
Look up Vyper340 and get some of his Ringo. It will clean the bottom w/o tearing up your gel.

Word of caution,

I used muriatic acid to clean my boat out back of my business one time, a county environmental officer happened to drive by and ripped me a new a-hole and threatened all kinds of fines. This was because the parking area drained into the storm sewers.

Star-brite hull cleaner, 8 bucks at Walmart...

Toilet bowl cleaner works as well and usually doesn't have the bad fumes.
The Works toilet bowl or tub and tile cleaner is fast and easy watch the nastiness run off the boat.

THE STAIN DOES COME BACK FASTER. IVE USED MORE WAX TO PROTECT IT BUT THE WATER LINE COMES BACK. WHAT NOW?

Chrome wheel cleaner. Spray on and imediatly hose off. Works great.

Tilex mold and mildew remover, bathroom cleaner, comes in a nice spray bottle.

I always use oxalic acid, It definately works awesome. I buy it from the local paint store in 2lbs containers ($10-15) and mix a few of tablespoons with a gallon of hot water. Just spray it on and wait 30secs and repeat a couple of times to cover all stains, then rinse off. Really no need for rubber gloves either.

http://en.wikipedia.org/wiki/Oxalic_acid

Compared to the muriatic acid it sounds a bit less aggressive
http://en.wikipedia.org/wiki/Muriatic_Acid

SLX is a cleaner just for this sold at boating stores works great