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.
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'75 Cuddy with '00 Johnson Ocean Pro 150 horse
Benny
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