Try these alternatives:

“The icy water grabbed at me: it almost took my breath away.”

“The icy water grabbed at me, almost taking my breath away.”

“The icy water grabbed at me and almost took my breath away.”

It depends on the effect you want it to have.

Grammar

The colon would be put into good use here because, although the clauses are related, they are separate. A full stop would be too much of a separation and a comma is too little. If you find the principle difficult think of a colon as introducing an explanation (in dictionaries they often have “word: definition”. You can also use a semicolon (;) to join two statements which mean the same thing.