As others have said these are well known obstructions. To quote Rod (Approach from the north) "On rounding the protecting mole, two rusty 44 gallon oil drums (sometimes painted red) are the buoys marking the underwater rocks and shallows on the SE side of the canal entrance, though they cannot always be relied on to be in place."
Still a scary place when the afternoon wind blows down and there is a lot of traffic!