As you move South along the coast, you will find sharks. They can get crazy as you get near seal rookeries South of Durban, but up towards Tanzania I was told the warm currents keep them away. I think those are the same currents coursing through North end of Madagascar. Or maybe it was cold currents? Whatever the currents were, they kept sharks away. Anywhere you have seal rookeries, I would not go in the water.
I really love Tanzania, and could have moved there to be honest. I still have plenty of good friends back there that I talk with weekly. The general population is extremely poor but you'd be hard pressed to find a happier more positive populace. I got caught up in a few riots but was never in any danger. We'd often stumble home across town in the wee hours messed up on konyagi and the only ones who ever stopped were people offering to take us home. One guy I worked with was involved in a coup attempt years earlier, he owns a small airline there now. He definitely helped us smooth things out when we needed "connections". The one thing that really brings the place down is the garbage, it's everywhere. Not much pride there unfortunately. The locals seem happy with the new government and cracking down on corruption. Kind of messy but it's one of my favourite African countries by far.