Good point. Legos are made of acrylonitrile butadiene styrene, a type of plastic. Unfortunately, this type of plastic does not do well with salt water. It can deform and possibly just destroy these Lego bricks. Including the fact that these Legos have been in the water for 25 years, they have probably softened to the point that stepping on one would immediately break it. Most fish have also eaten this, probably mistaking it for small life forms, lowering the chances of stepping on one, or even finding it.