Hi.
Like you, I come from the area (keep a vessel in Chichester Marina, often in and out of Portsmouth).
There are a number of threads about it, but basically, the whole Solent environment was modelled using Chart Datum (i.e about 0.5 - 1.0 metres lower than Mean Low Water Springs). Hence the lack of depth. So, think of the lowest tide you've ever seen in Portsmouth. Then take anouther metre from that.
It was complicated by the fact that the land height is based on Mean HIGH water Springs. So about 5 metres (15 foot) of water/land is missing. You'll get the Hovercraft into Portsmouth Harbour but not a Ferry.
Shame. I understand there are plans to fix this at some stage. But it is a HUGE amount of rework.