jimmy2liter, check out my thread under "Modify!" section entitled "The Stereo Journey Begins". It is sticky'ed at the top. I am currently doing just that, adding a subwoofer and a whole lot more, including tower speakers to a 2006 Utopia 205.
If your shop says they cant add one, they are idiots! you could add one under the back benches, probably could do up to 6, 8" subs there if you don't mind loosing your storage. You could do 2, 10" subs in the walkway up to the bow, on the sides towards the floor. You will see there is a nice big flat spot on each side. Pull up and then out on your bow cushions to remove them. behind them is a big opening for access. You will see there is ample enough room to build a box in there. Any installer worth more than a pile of dog dung could put a sub box in there by building the box. Then cutting out the the opening big enough to slide the box in until it is flush. Use some brackets to secure the box. Seal it with some good silicone, trim with some nice aluminum or stainless and you, my friend, have a sub on your boat! If they are really good, there is some good amount of area to mount a sub under the passenger dash facing towards the floor. I was going to do this but couldn't quite get my 12" to fit. I know you could do a slimline 10" with box in there easily! You will see on my thread how I am doing my sub and that is another option as well.
That seriously irritates me that they could be that non-imaginative! Hope this helps. Any way I can help, holler.