The showers by the finish line are a shower head mounted on a pvc pipe. Good for cooling off, bad for dropping your pants and changing. (well, unless you want to end up on www.sotar.us for life.)
The expo page notes that there are free showers for runners at Schryver Park pool. Just show your bib and they will let you in. Full dressing room there. (I have not personally used it, but it looks decent).