I had a very similar situation - a concrete box that serves as a duck blind which sits mostly submerged. Poured in 1955 we have killed a lot of ducks out of it. Anyway it developed a crack and leaked. About 18" of water inside per day had to be bailed out to sit in it.

I waited until the outside water level was about gone then brushed the area hard with a wire brush, chipped a bit along the crack to widen it, and cleaned it with muratic acid. Then applied hydraulic cement. Its held well for three years now, including ice inside and out.

I would have used a pressure washer if accessible.

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