my "orders of magnitude" alarm is going off here. There's no way that the implosion of a fairly small sub is going to be heard in Boston even if underwater. Something doesn't make sense here.
http://resource.npl.co.uk/acoustics/techguides/seaabsorption... -- here is a quick calculator for sound loss by distance. I think actual geography is important too but from understanding whether or not it's hypothetically possible, it certainly looks like it.
For example, at the default inputs we see .061 db/km absorption. This is at 1khz. Higher frequencies attenuated more and lower frequencies less.
I have no idea what frequencies an implosion generates, but given that, a sound at 120db might still be 60db 1000km later. Certainly seems possible and in fact given what we have seen from the US Navy (detecting imploding soviet subs in the middle of the pacific ocean) it seems totally possible to me that this small sub could be detected if microphones were places in quiet spots offshore of the continental US and Canada.
I think we don't have enough information to rule out that this was detectable.
