My kids were into renting the small watercraft every summer over the last few years. I don't recall ever getting 30% off with my AP at the marinas (maybe 20%), and I'd ask every time which discount gave the best (DVC, AAA & AP) I'll have to pull out the old pile of receipts I know I have around here someplace to see what they did give me.
Update: My daughter was the one who gave them her card last summer and as she recalls, between the three the discounts were 15% (AP) 10% (DVC) and 10% (AAA)... so no changes there within the last 6 months.