The one that you are reffering to is just Southeast of the 1MC bouy and is just below the surface. I have seen sand on this spot but not the concrete (not saying there isn't, I've just never seen it). Typically when running through this area I will take 1 of 2 approaches because of the many sand bars in the area. 1 I will run South past the 1MC marker approximately 1 mile before I cut over to the East. Or I will run through the Middle Channel to roughly the R1 bouy (on map) and then head South. Reverse this when coming from South to North. Keep in mind that wind can move these sand bars and create new ones at anytime so it is wise to give this area a wide bearth. Also when running through this area I pay close attention to any changes in water color (indicating a sand bar) or waves that may be breaking differently than the surronding water.
Sand Island is often easy to spot as it out of the water, there are often times boats beached on it, and I believe someone has even put poles with white flags on it(???). But I still see boats, mostly large cruisers run over these sand bars all the time oblivous to the danagers. Carefull guys and thanks for the heads up H20. I'm just glad the area you are talking about is one I knew of and not something new that I could have been in danger of hitting!!