It is interesting to find contrasting images from the old and new testaments linked. One possible explanation is that the God of the old testament is the same God in the new testament: Jesus. When he says, "Don't you mourn....Pharoah's army got drowned" perhaps what he is really saying is that Jesus (God) is not dead, look how he triumphed over Pharoah. Remember, he's the same God in both old and new testaments. It is also interesting what you said about Moses being the symbol of liberation. In the old testament God liberated the Jewish people when Pharoah's army was drowned. In the new testament God liberated many more people from their oppressor, in this case sin, when Jesus 'died' on the cross. Both were acts of liberation and may be the reason the two are linked.