If you mean your own forces:

Go into Options --> Interface

Make sure that "Show Full Fleet Battles ONLY if both sides are Fleets" option is disabled.

I think that should do it, and you'll see every battle you get into whether it's fleet versus fleet, or ship versus ship.

If you mean fights between ships not involving your forces, I don't think it can be done.

Oops, I think I made a mistake, though.

In order to watch single ship on ship battles, you may need to enable "Always Watch Player Ships in Full Battles."

This option is found in the Interface menu, too.