I thought using the diving bells would be really fun in this game, but between getting air and getting sharks on me the instant I swim into the open, it's no fun at all. In GTAV diving into the ocean was amazing and there was tons of cool things to find. Here it's just a chore. I can't imagine why they would do this.
Playing: Assassin's Creed IV Black Flag Waiting: R&C Into the Nexus, inFamous Second Son, South Park The Stick of Truth, Watch Dogs
I never had any issues with sharks? Just treat the underwater missions like a stealth mission and they won't bother you at all. Move from weed patch to weed patch, behind or through wrecks and their sensor will never fill up high enough to actually target you unless you purposely swim in to his mouth.
Most of the dive spots that lead to a bandit hideout, do not have sharks.
Also, the good thing about diving is all the collectables save as soon as you get them. So, if you die, you lose no progress and just continue collecting what you havent got.
I did not like the diving parts myself as i have a stupid phobia of underwater creatures and such. Sharks arent really that bad, but its still unnerving to me. That said, they werent too bad. I just swam as fast as i could and just took the damage when i couldnt avoid them. As i said, you lose nothing if you die.