[Targan]

I've arrived a little late in this discussion and much of what others have said is true so I won't double up. But I will say this: depending on the nature of the timer and how you want to set the explosives off you can use just about anything explosive as long as you put it in a plastic bag and seal it. The steel hull of a barge is a pretty tough item, if the explosive isn't tamped in some way and/or is not a shaped charge it will have to be a pretty significant blast to penetrate.

In my campaign the 2IC of the PC group is a USN SEAL officer, Tadeusz Jones (Polish mother/Welsh father), and he led a very exciting mission at the end of the Pirates of the Vistula/Ruins of Warsaw modules when the group wanted to steal a Shershen-class torpedo boat from the bad guys. To create a diversion and reduce the risk of being chased Jones and the group's combat engineer put together several self-forging shaped charge underwater limpet mines. Their design was quite clever I thought - for the copper self-forging plates they used the sort of copper bowls you find in many kitchens for fluffing up egg whites, and to attach the mines to the hulls of the target ships they used magnets from hi-fi speakers.