There are two actual Ponquoque bridges, the new one and the old one. You crossed the new bridge to get to the former Foster Road. The old bridge is where you will probably dive, although nothing is stopping you from diving the new bridge. Don't get caught in the channel between the two bridges, however, because it is considered a channel and it is illegal to dive in a channel in the town of Southampton. With that in mind, there is usually good parking at the bridge, but you will have to do some walking in order to get into the water.
This spot is upstream on the Manasquan River. The diving area is rather small and not very deep. Visibility tends to be poor, and this is worsened whenever an O/W training class is in session, which is often. Also, the current under the bridge becomes very strong, so slack water is essential. For a long time, a dredge barge has been anchored in the channel, but otherwise, there is no boat traffic.