Hello once again. I know nothing about underwater photography, so I couldn't even guess as to the height of this.
It sounds logical that this could be it.

Peter had this to add to this story today:
"I would say that if it is a ship mast then the specific ship type and even its name can probably be located because the mast is very distinctive in appearance. To give some idea of the weight used to hold the masts in position - the Australia 2 yacht that won the Americas cup in 1983 (?) had a 20 tonne weight attached below the mast. Most ships have this sort of weight to hold the mast in place and help the ship become more buoyant. Older ships would have had something although probably not that heavy it would help the mast fall as a spike. Interestingly, the majority of shipwrecks are sitting upright on the bottom of the sea or are slightly rolled but not totally onto their sides.

regards
Peter Ness"

And thank you Peter, if you're reading this.