I've gotten marine plywood at John R lumber at 7 mile and I-75. I forgot who recommended it to me. They had a lot of specialty marine stuff, plywood, mohogany, etc. I found them as this on the truevalue.com site-
Public True Value Hdw.&lumber
1001 E 7 Mile Rd, Detroit MI, 48203-2161
313-891-7125
Public True Value Hdw.&lumber
1001 E 7 Mile Rd, Detroit MI, 48203-2161
313-891-7125