"What are your thought about a fiberglass hull?"
I wouldn't worry about it at all. The overpressure is so small it is very unlikely to cause any damage. The amount of explosive in a few inches or a foot of detonating cord is very small. It works because it burns so quickly and creates a shock wave that is very sharp but of low duration, a property called brisance, which is just the opposite of what you would want to blow the bottom out of the boat from a charge in the water.