I used to be the Bridging Equipment Storeman for 2 Combat Engineer Regiment and then later on, the Engineer School (between 1998 and 2002).
While all of the FBE, LFR, APR/APB, etc were long out of use during my tenure, some of that kit was still sitting around at both Brennan and Swan Lake. There are still some Modules sitting on Trailers at Lauvina Woods here in Gagetown.
There was an urban legend about an "old Jeep that went down" in Cultus Lake, not far from the old Boat House. I can't say i've ever heard about the FBE you found.
I'll ask some of the older Combat Divers if that was put there as a Training Aid for the Bubble Heads, or if it truly sunk during training and was abandoned. I'm leaning toward the first, as the Divers have pulled crazier things than that up from the bottom. Either way, its a great find.
As far as the Pin is concerned, my guess (and its completely a guess) is that it was a Release Pin that was pulled with a Rope to release the FBE modules from the Trucks or Trailers into the Drink....
On a side note. We actually managed to get a "Rotorque" BBE running while I worked at Swan Lake. We had it on the water for a Summer although i'm not sure its status now.
You may want to contact the CF School of Military Engineering here in Gagetown to discuss your find:
CME.research@sympatico.ca
Scotty