I'll second Andre on this one. I keep my RIB at Littlehampton and have been caught out a couple of times.
If you know the river exit you will have seen the wall Littlehampton side which is only visible at the lower end of the tide. If this wall is visible on its seaward end then you have about 1 metre of water at the entrance.
Generally this will only be for about an hour on the bottom of a neap tide or for a couple of hours on a spring.
The Littlehampton Port web site will also be able to help you and they have a good seaward looking web cam so you can see the sea state before leaving home !!!
Its saved me a trip a few times !!!
Happy Ribbing !