Absolute, unthinkable worst case scenario:

A friend or fellow ice-fisherman near you goes under, and doesn't come up on his own.

What do you do?

This would really send you into panic mode, but it's best to think of it now, and prepare yourself mentally for what you would do.

I've never prepared for this.

One plan would be to keep a section of rope, with a brightly colored, sinkable object to throw down the hole for him to grab onto, so you can pull him up.