The only time I have encountered a (bull) shark that was acting aggressive, swimming fast, pectorals down, trying to get behind me I was with a buddy. We were submerged, well, it was at the Destin finger jetty.
I think the only time that a solo diver would perhaps be at any more risk would be during a surface swim transiting to and from a dive site.
Not hardly an expert on shark behavior, but it does seem, just as any predator, they pick out their victim and once initiating an attack will seemingly ignore others in the water while pressing their attack. Which is how other people are often able to fend the attack from the victim without themselves being hurt. So, if this is the case, maybe in that odd and rare instance, a buddy would be useful.
I think the barge is called the Lucy, about four miles east of the Destin/East Pass in about 60 feet of water and about .75 miles from shore. In any case, way back, I launched myself solo from the park just ashore from the Lucy and swam out there. I was in the water about three plus hours. After doing the dive, I surfaced and began the long swim back to shore. Suddenly something was hitting me, here, there, everywhere. I looked to see thousands of baitfish apparently being schooled by kingfish. Me, being the only structure, they were holding on me in their attempts to avoid the kingfish sweeping through them. The dense schooling went on and off for a good portion of the swim. This was most disconcerting at the time. I am not sure, had I had anyone else stupid enough to swim a mile offshore and enjoy the adventure with me, that it would have been any less so.
I had a similar event a few years back at the Commercial Pier dive in LBTS out to the deep reef. It was small to medium size tarpon this time. I went through a greenish area on my swim back in and the fish were schooling all about me, hitting me. I think the greenish water might have been a result of low tide and some freshwater maybe. Not sure.
N