This is a complete guess but I'm assuming Neptune Marina is on the Shark River Marina. If so I believe they are currently conducting the dredge operation in the north channel and if I had to make a wild ass guess maybe the polluted waters marker you saw has something to do with the impact the dredge operation is having on kicking up hazardous sediments from the dredge operation. That's a complete guess but maybe the reason for the signage. Would be curious to know what you end up finding out.