All together too many things may have contributed to the sinking of the Fitz to prove any one thing was responsible. If I had to guess based on what I saw on the history channel and what I know of boats I'd suspect two things.

If she hit bottom hard enough to hog the hull off Caribou Island her keel was compromised if not broken. Then traveling in a following sea
with close wave caps and a deep trough (A common enough cause of sinkings in The Great Lakes) the unsupported mid section would flex the other way and crack the ship through the middle.

No matter the pumping ability this probably wouldn't be survivable.

Don