It could be worse.
At least it wasn't heavy
oil.
Diesel evaporates readily and I have been told US warplanes routinely dump JP5 (kerosene - it's in the purple colored pipes on carriers

). If this is untrue I stand ready to be corrected.
I, for one, consider dumping empty 55 gallon drums at sea in thousands of feet of
water preferable to trying to dispose of them on land. They will disappear in short order. Better yet, preferable to turn them into a
steel pan instrument. But steps should be taken to allow any residual diesel to evaporate and to make sure the drums sink, i.e., cut the drums in half with a grinder before disposing. They'll make a nice
fish condo until they rust away.
That said, even carrying 55 gallon drums of diesel on board is bad form and if a mega sailing yacht cannot cross an ocean without that reserve it's contrary to all cruising
customs and mores I respect. I think they need to do some sailing and turn off all their needless
electrical goodies including AC,
satellite TVs, etc., etc., and underwater lights.....
Dave