@TexasAggie2009 My pool is 18 years old with its original flagstone on the coping and water tiles. I had it sealed back in 2018, I cannot say what was done before that as I am 3rd owner. However, it is in great shape and I do not notice any "sand" debris in the pool. I cannot relate it directly to fully submerged flagstone but clearly half my flagstone water tile has been submerged for 18 years with no noticeable issues. I even brush the flagstone water tile routinely as part of my maintenance so it does see some external forces on it without any ill effect.The one thing I notice is there is a ton of what looks like flagstone, including the entire tanning ledge. Flagstone is pretty notorious for flaking off into the pool. I would be nervous about going with that for your coping, wall, and especially submerged under the water on the ledge.