Hmmm...
I don't know the anatomy of a shark very well. The Great White has a brain the size of a golf ball. I'd have to guess where the vitals and heart are, somewhere in the chest cavity, head, gills, spinal column, brain of course....
Thick, solid muscle. I don't think they have much skeletal bone though. Mostly cartilage. I think you'd just have to get really lucky, or do a massive amount of damage to muscles and organs to scare it off.
I'd want something in the .30 caliber semi-auto variety with a lot of bullets. Probably an AK47 with HP rounds and 30 round mags. Or perhaps a semi-auto 12 gauge with at least 8 round mag tube, stoked with 1 oz slugs or buck shot.
Then again... huge bears are like 10' tall and 1,500 pounds of muscle and bone. And they are quite resistant to smaller calibers. I've read stories of bears taking multiple 7mm magnum hits before dying.
A Great White can grow to 20+ feet, and weigh 7,000 pounds. And the bullets may need to travel through a foot of water, which will bleed off energy quickly. A bullet may need to be able to reliably travel through some water, and several feet of muscle and fat to hit a vital organ!
So, maybe even step up to a .308 semi-auto like my M1A or FAL and 20 round mags.
It would have to be in the 16-18" barrel variety, for quick mobility.