Answer:
The construction of a new pier.
Increased non-motorized boat traffic would not really do anything, because the ocean would wash it away like always.
Recreational fishing wouldn't pose such a threat, we have been doing this for thousands of years.
Humans swimming around a pier wouldn't pose sucha threat either- they will not kill anyone.
But the pier would release a lot of sewerage, oils, and more harmful things than all of these other things combined.
Hope it helps!
