Horizontal crack behing pool spa (w pics)

So during a short freeze on of the tiles fell off. I then noticed a horizontal crack running along about 6 tiles. I then removed one tile and found a crack on the wall behind it. I'm going to remove as many tiles as it takes to find out how long it is.


There is a crack on the top surface of the jacuzzi that could have been the cause of this (I just bought this house in November so I think it was neglected). My guess is the a have to dig the crack out and build up the surface with ez patch no 1 (plaster I think)?

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Ok so the crack starts from the tunnel that flows into the pool an goes about lets say 10 long. At one point the crack looks deep, but it does seem too wide. I will post a some more pics tonight.

What could I use to fill the crack? Is this crack considered (heavened?)