You do not need a certificate but you are required to sign a declaration that you have read the BSS booklet and that your craft complies with its requirements in every respect. You can see the exact wording on the application form available for download.
Well if you want to split hairs you need a BSS on the first 1000 feet ( ish ) of the tidal part , because that is controlled by the EA as far as the boundary stone below Teddington Lock . /forums/images/graemlins/wink.gif