I don't think the regs for anchoring under a bridge are written very clearly. After doing a little research, it seems this only applies if the bridge is in a security zone or if you are blocking channel traffic. Specifically, the one example I read is the big bridge in Tampa Bay (Sunshine Skyway), only the main span is listed in the security zone and may not be anchored to or under, there is no mention of restrictions for the rest of the 4 mile long bridge. Bridges are a heightened security area, and you are more likely to be given a check, but it doesn't appear to be illegal. Next time I see a Lake Patrol, I'm gonna ask for clarification.