I think the sailboat theory is sound, but I've heard two others.
The first is that it's awkwardly piled concrete from the old drawbridge. If so, I'd think that this could be very dangerous to dive, but I'll let you be the judge. I also have a hard time giving much weight to this theory; why would they dump this concrete right on top of the new bridge piling foundation? That seems like a nightmare scenario for any structural engineer.
The second is that it's actually the remnants of the OLD, old bridge....a wooden (I think) structure that pre-dates the drawbridge (not the RR bridge, which as you know, is farther upriver).
"Don't talk to me about naval tradition. It's nothing but rum, sodomy, and the lash." - Sir Winston Churchill