Good luck and be careful not to draw too much attention! Oh, if you just had a magnetometer or a really good underwater metal detector! A cannon with identifiable markings could lead to the origin of the ship and who knows, maybe the name of the ship. Due to the weight of cannons, they don't move very well on the sea floor even during hurricanes, so if you find more than one, then you may be on top of the shipwreck and the Mother lode.
Oh and I almost forgot, if you haven't already metal detected from the water's edge to several hundred yards inland, don't neglect to do so! In the Florida Keys, pieces of eight coins from a shipwreck on the Atlantic side were moved by the many hurricanes over 300 years and blown into and across the sand dunes and some ended up on (that should be in, right?) the sea floor on the Gulf side. A friend found 2 pieces of eight (don't remember the denomination) at this spot, then later working with a partner, they found over thirty 1, 2, 4 and 8 Reale pieces on their first trip together. They found more on another trip but then my friend found that his partner was sneaking back at night, digging more pieces of eight and had totally destroyed the Sea Oats in the sand dunes. Due to this, it wasn't long before the authorities put the site off-limits.
Frank