Discoloration on the waterline
 

Any advice on how to remove the brown discoloration on the fiberglass at the waterline?

Depending on how bad it is there are several things that work but might be too aggressive for your taste.
CLR may remove it if it has not been on very long.
Muratic acid will do the job but it will etch the gel coat if you are not careful. It will also damage the trailer so take care not to get it on anything else, including you.
Vinegar is slow but fairly safe.
You could also buff it with rubbing compound or water sand it with a fine sanding sponge block, then buff a shine back on it and wax.
I am sure there are some squeeze bottle things on the store shelf I've not tried supposed to do wonders.

starbright hull cleaner

Use Sno-Bol Toilet Cleaner. Use full strength and wear rubber gloves.

^This used to be the ticket until they took the hydrochloric acid outta it. Really ticked me off cause I agree it was the best!!

Now I use wood bleach, does a good job......Just do smallish sections at a time and dont leave on for long

I have used Y10. Does a great job of removing the waterline stain with no scrubbing. You can get at most marine stores. It costs about $10.00. One container last me about 4 or 5 seasons.

George

I second the Starbright Hull cleaner! Def does the job

I use whatever the brand is that Walmart sells.....cant recall at the moment but it works great for the brown I get.

MaryKay Off and ON Hull cleaner. Will work like nothing else.. USE RUBBER GLOVES and GOGGLES AND WORK OUTSIDE as the fumes are toxic.