Depends what the cargo ship is doing really. If it's coming alongside/leaving the wall then you could certainly foul the prop- leading to loss of manouevrability. However, they will almost certainly have tugs in attendance and, apart from the cost of getting divers down, general annoyance, and potential delays to cargo, not massively massively serious.
In open water while underway, you could foul the prop on something like discarded pelagic/purse seiner lines (big nasty fishing kit) but this is highly unlikely to immobilise. For a start, most merchantmen aren't single screw, and can cruise along happily enough with one prop inoperable if necessary.
Warships tend to go around on one screw as a matter of course if they're in cruising watches.
On the Chichester 58/59 heading to Capetown across the Indian sea when it was noticed the port engine was running slow.Shut it down and ran on one for a few hours,finally stopped ship and sent the whaler away with a crew and a couple of divers.
I wasn't in the crew but looking over the stern when.....!3/4 of a bloody great shark floated up!Everyone on the seaboat crapped themselves,the divers filled their suits and we had a good laugh.
Seems it had jammed itself in the A bracket.
I never thought that would slow down a prop but it did ,enough for it to be noticed and checked out.
I was lookout on the bridge when first reported there was much scatching of heads and the Eng.officer lost pounds running from engine room to bridge.
It was a massive bugger, even the part that floated up.No wonder the divers were pale.