Saving in battle is disabled on purpose because it is buggy.
Some things that are important in battle cannot be saved, so when you reload, you won't enter the exact same situation.
For example, your sails are automatically set if I recall and your cannons already loaded.
This happens when not in battle too, but then it's not so much of a concern.
Well, I'm sure it's less of a prob to have a slightly diff setup upon reload, than to lose the entire multi-ship battle due to a crash on the boarding scene of the 3rd ship or somesuch... :/
Maybe an autosave everytime a ship surrenders, is sunk or boarded? SO you at least get some reference point if you crash?