Ocean Leisure is OK...it's just a West Marine with people speaking a different kind of English and the prices in pounds versus dollars. There's a little, crowded, messy store in Covent Garden - at the end of a block - Arthur Beale's. Been there for 400 years! And some of the stock has been there 400 years, too!
You can go to West Marine here in the states...see London instead!
The boat ride from Westminster to Greenwich is lots of fun. The naval museum is fabulous when you get to Greenwich.
Walk up the stairs to the top of St. Paul's dome on a sunny day - great views of London.
If you're a Churchill fan (as I am) go to the Cabinet War Rooms and see the terrific Churchill museum there (along with the War Rooms - they simply shut the door after WWII and then opened it up about 30-some years ago.. Go to Churchill's country home (Chartwell) outside London (a chore to get there, but wonderful when you finally arrive).
Go see a show in the West End. Go to the National Gallery.
But whatever you do, go to the Sussex pub in Covent Garden, get an outside table and watch the people - the number one stop for us when ever we get to London (we've been 11 times in the last 40 years).