The marina could or could not have paid him to provide "security" to tell people to slow down. But if he was on official CG patrol, he's acting on behalf of the USCG, not the fuel dock. If the fuel dock asked him to slow people down and he was not on official patrol, but using his "coast guard aux" credentials on behalf of the fuel dock, then there may be an issue with this aux member misrepresentation of his/her duties. I recall an incident similar to this happening when I was at Gru Ohio River back in the mid 80's.
Maybe rules have changed since then but aux members unless authorized and in company with active duty CG personal have no enforcement authority. That's NOT what they were chartered to do. Like I said, maybe the charter has changed but it seems unlikely.