bottom trawling - scrapes the bottom destroying coral gardens etc.
Longliniing - incidentally catches animals and birds, including turtles and albatrosses
pollution - sewage and industrial - treated as a rubbish pit
desal plants are a new threat.
Channel deepening and dredging off coasts
CO2 emissions which concentrate carbon in the water which screws up the pH - and causes ocean acidification
garbage - especially plastics which sea creatures eat and which then kills them.
200 dead spots in the ocean.