Flagstone is a very generic term. Some are hard & durable, some soft & sandy and others are flaky. Ours is a little flaky, much less so now that it's about 2 years old. Salt has not been an issue but you occasionally see warnings floating around. The concentration in the water is low but you could potentially have a build up where it splashes out and evaporates repeatedly. I personally doubt it will have any long term effect so we went with stone.