The sharks that feed on bonefish are going to get enough of them to keep thier belly full whether they take if off the end of a line or not.
Actually, I believe few sharks have the speed needed to nab a healthy bonefish cold. Cudas, maybe, if they're lucky. On the other hand, a bonefish that's tired from a fight is easy game if it can't make it back to the school... if it even can.
Without reservation (nevermind my past, greedy behaviour) I can say that if a shark chases your hooked bonefish there is only one solution: break it off. Fast.
In point of fact those sharks that frequent bonefish flats and quickly nab a hooked bone have learned
this behaviour. Sharks are much too cautious to rush into such a situation, especially with humans standing around. (Of course, I'm talking about your average lemon sharks, here. Bulls may be a different story, but luckily they are rare.) I think that the spear-gun is a great idea... if you take the tip off the spear. Sharks have famously thick skin and seem to be quite hard to hurt. A solid blow from the blunt point of a spear may be the answer. By no means do I suggest actually harming sharks (they are truly magnificent animals which deserve our respect and attention) but a regularly fished bonefish flat sets up an unnatural situation that in itself may throw the local eco-system out of whack. This is always the case when one species gets an unfair advantage (or disadvantage, for that matter). I'm merely suggesting that if sharks can learn to steal bonefish, they can learn not
We all want to catch bonefish, as do our guides. Don't fall for their advice to just throw slack at the bonefish. Do your conscience and the fish a favor: pop it off.
Oh, yeah, and don't jump on sharks if you can help it. It's a great ride, but certainly not for everyone.