No seriously I mean lots of debris!
Pallets, railway sleepers, huge tarpaulins, trees, & load of wine/beer bottles.
The really big bits seem to be down stream of the barrier, but even upstream of Woolwich there's loads to watch out for. I'm sure when they say the water is relatively pollution free now they are correct it just looks pretty grotty!
Last time we had trip from the isle of dogs up to Putney we counted 14 tennis balls and 5 footballs too! At least they just bounce off:-)