to me it sounds like its time to pull that motor for further inspection...it sounds like the freeze plugs did their job, but why even chance it? do you want to risk sinking your boat over a $20 set of freeze plugs? Don't want to sound like debbie downer, but definately a precaution i would take!

Replace the ones that popped out, the pressurize the cooling system to 20 psi.....if it holds, run it.  If it leaks, fix it.

Replace all plugs and press test...plugs are less than 10 bucks people.  use brass only 

I think brass plugs used to cost $10 for a set,, don't think anyone is selling them for that still? 

When you do that pressure test after replacing the popped plugs keep pressure on for at least 20 minutes or so for a good check and then start your engine and run for awhile and then check oil for milkshake, I have seen blocks and/or heads with very small cracks that don't leak very much if at all till things warm up by running, I hate to say it but if you had such freezing to push out a couple freeze plugs expect cracks. Here in Pa we get many boats with problems from freezing every spring and sometimes they don't show up till after running for awhile.. Roy