One last thing for now.
I did pull this off the weather.gov site.
It's the climate and weather conditions for the Palm Beach County Area on the 24th. While not a "marine" forecast, it might give you some idea of the conditions afloat.
http://w2.weather.gov/climate/getclimate.php?wfo=mfl
According to my resources down there, they had charter boats coming in as the kids were going out, reporting that they were getting "beat up." And these were 40'ers and such.
As for the possibility of a engine cover coming off when the boat overturned, anything is possible. What I can say is, most of the engine covers I've seen in the last many years, all have pretty secure latches that have to be removed to remove the cover. Not real likely to just pop off.
Having been on the scene of several overturned vessels over the years, I've never seen one that came off unless there was significant speed involved (i.e.: stuffing the boat at speed, a roll over, etc.).
I would have to agree with the engine failure scenario in this case.
It would have caused the kids to remove the cover, and likely given rise to both of them being at the stern of the boat while DIW. From there wind and wave action could have been the cause of the overturning/swamping.
Then again, this is all speculation.
LE will not "close" the case.
They will inactivate it when they've run out of leads, but if anything pops up again, they'll reactivate it. If they're never found, the case will more than likely become another "cold case."