I understand what you are saying Mark. But, the owner of the construction company specified that he wanted the explosives from his site used. If different quantities of explosive should be needed for the pier on land, vs. the one in the water, those are just two calculations.
If the players want to get cute, it is a simple modifier for different types of explosive. If they use too little, it still stands; too much, and damage occurs throughout the city. Yes, there are all sorts of parameters possible as to the placement of the charge. But, with the rules, in general, using so many shortcuts and generalizations, surely that is overkill. There is so much generalization in the rules, that it's just the idea that counts. Surely, moving the fishtank without the water would be a formidable statics (or dynamics) calculation in and of itself, if one were to go that far!