You need to determine whether it is indeed a crack or whether it is a "check". Larger pieces of wood will naturally develop checks and they are nothing to worry about. I remember the first time I saw the checks in the 80' mast of a schooner and was convinced that the rig would come down when of course everything was perfectly normal.
I would suggest watching the crack while sailing and see if it opens more. If it does when you are sailing hard, then you should do something about it.
If there is in fact a problem, it would be wise to figure out why. It could be that the compression post is simply not large enough. It could also be that it has started to rot(if you try to pull a little bit of wood out from the crack, what does it look like?)
As for repair options, you have a couple. The post will still be just as strong in compression except that it will be more likely to buckle so it is buckling that you need to prevent. In addition, you need to worry about rot since there might already be some or your repair could trap water and cause it. Given the size of the crack, packing it full of epoxy will be quite difficult. The problem with running bolts through it is that you create stress concentrations. Another option is to fabricate a collar that goes around it and clamps it together. As long as the post was big enough to start with, it can take the compression so you just need to hold it together. You don't even need to hold it together along the whole length, buckling is proportional to the square of the unsupported distance so just attaching it at the middle will make a big difference.
From your description, I am not even sure there is a problem. Someone who actually understands wood can help you with whether it was caused by stress or whether it is natural.