[@KoL]

My calculations on how devastating the ice burst should be to someone in the water are generally resulting in two conclusions-

Either A: "The pressure is mostly relieved by ice expanding upwards, Miyu is fine, but trapped."
Or B: "The pressures are so intense (getting insane "the ice is now a metal" level pressures), Miyu is crushed to death instantly, because steel would behave like wet tissue paper at those pressures."
All dependent on how fast the icy burst is.