What attacked the sub in the beginning?
From a previous discussion, what attacked the sub in the beginning that Statham was rescuing people from because only later do they inadvertently create a warm hole for the sharks to escape through?
Later when the blonde lady and 2 fat guys go down in the sub through the ice cold layer they engage the huge creatures for the first time and then the 2 sharks escape.
It could have been explained as something else like the sub inadvertantly going through a minefield and leaving his crew there was the only way for Statham to save the rest.