It's quite possible and always a worry out there particularly this time of year and the Humpback Whale population seems to be steadily on the increase. I have come close to Whales particularly at night and you just never know where they are going to be so I guess we are going to hear of more of these types of encounters. Have to ask yourself when travelling around on the ocean at night or even day "Are you feeling lucky?"
Highlights the need for a floating grab bag with all your lifesaving essentials in it, especially a Hand held VHF and EPIRB, obviously no time for anything else plus good idea to wear the life jacket with a torch attached at night while traveling.
Being inside an upside down boat at night would be a very scary experience, I imagine pitch black and not knowing where you are or whether you are going down, where your emergency stuff is???