As you go along Southsea Beach towards Pompey, roughly level with the end of the pier, but about 1/2 mile before you get to it, you will see two huge markers sticking out of the sea about 30'-40' apart. This is your channel.
As Duncan says, once you've seen the blocks at low tide you will keep well clear.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club