And I wouldn't use a jackhammer, you'd be transferring a lot of vibration from the chain to the adjacent hull/structure, potentially causing cracks and/or other problems.
Sea Buzzard,
Evaporust! You can get it at Walmart ~$10/Gal. Spray bottle it on & let it soak. Biodegradable, Non-Toxic & it works.
Star
I think these two suggestions used in combination make a lot of sense.I don't think it will be as bad as it looks. If you can apply some light force to the chain (say an anchor hanging off the bow) and just follow the chain with an air chisel, it will release bit by bit. The weight will pull it up and away as you release it, so it won't be in your way as you work. May take some time, but rust usually crumbles pretty well with some percussive force.
Prybar and profanity