Actually marinas like the one pictured or "canal estates" often create environments that become "infested" with pests and predators. Some of which for example contain bull sharks in plague proportions. Yes sharks live in the ocean but anyone living next to a canal teaming with them would conclude that there is an "investation".

Bull sharks are unique in that they can somehow retain salt from ocean / salt / brackish water which allows them to operate in fresh water. They're also more aggressive than other species, or perhaps just more likely to grab at movement in murky river water.

I remember watching a documentary suggesting that bull shark attacks might have been the inspiration for the Jaws series, after a spate of attacks in estuaries in the US in the years prior.

Jeremy Wade on River Monsters TV series also had an episode about them.