thank you all for your inputs! they are all very interesting!
so far 16 hearthlings can swim!
(i havent counted you, shubla, as your posts didn't claim your ability. if you want to be counted, please say so.)
as for surviving the real situation, we all know swimming for fun and swimming for survival is different, your health condition, weather, currents, what you have around (like life jackets, boats, some floaters, whatever), where you are, or who is around, all the little things change the odds you have. so, this is more about whether you would take a risk and jump into water hoping for survival or not (and i think you would likely try if you think you can swim rather than staying in the ship praying for the rescue team coming). even if you can swim very well, you cant swim on forever (like, in the middle of the ocean), you may not be able to find any floaters you could take a rest with, and sharks may come and get you! so thats that.
some (supposedly) experienced korean divers lost their lives while trying to rescue the victims at South Korean ferry wreck a while ago too, being able to swim well does not necessary grant you survival anyways. this time around, however, it is said that the most of the 14 survivors tried to swim and got rescued. those who were banging from inside the ship (i read there were 2 of them) didnt make it unfortunately. (the tragedy part is that many of them may not have had the chance even to get out of the ship at all, though.)
EDITED sorry i was misinformed, there were two rescued from the ship in the air pocket. (loftar, please make "strikethrough" thing!)