Helped a friend launch from the Porchester Sailing club slip - probably the only place I would want to land. It's just north of the castle and easily found with a couple of derricks / dolphins on the southern side of the slip.
If I remember correctly from the sailing club to halfway up to VT there is a sea wall. Away from the slip the tide definitely doesn't stay long and the mud / sand is really not pleasent. Slip is firm out to the bottom couple of hours around low tide.
Btw good images on Goole Earth of this area. And castle was good to have a look around if that's your thing.
All stated from the perspective of non local who might have possibly had one or two drinks in the Sailing club the evening before....