From the heavy corrosion, the round you found appears to be of WW ll vintage.
From it's shape it most likely was a high explosive round/tracer, not armor piercing, for a 37 mm cannon.
I would guess from an M4 cannon, which were used in the P-39 Air Cobra for air to air combat and also used on Navy PT boats, beginning with the Solomon Islands campaign, most likely by scrounging or salvage from P-39s.
Less likely, it could be 40 mm projectile (based on the blunted nose) used to defend a ship against air attack,
These anti aircraft guns were fired one after the other (not synchronized) and were nick named Pom Poms as one barrel would be forward while the other was back from the recoil.
US Fighters were typically armed with browning 50 Caliber machine guns , each machine gun being fed by a linked chain of
27 feet. Usually they were fired in short bursts to conserve ammunition. Giving them the whole 9 yards refers to firing all 27 feet at the same target
in a single burst - it was not derived from football.
Some aircraft were armed with the slightly larger 20 mm cannon which is to small to be what you found.
The advantage of the 20 mm was extra range, the disadvantages were more weight and a slower rate of fire.
live round 
I heard that there used to be military target practice near the SA pier.
