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.
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