Brighton Marina the right way
If you're going to Brighton Marina, it's best to take a bus. That'll take to directly to the good bits, and the boats.
Brighton Marina probably looks quite nice when you enter it by bus. The bus takes you to the key places in the Marina, like .. the boats and the water. People who first see the Marina this way often really like it.
Brighton Marina the wrong way
On the other hand, if you approach the Marina from the beachfront, say, by taking the Volks Railway to Black Rock and doing the last couple of minutes on foot, then first impressions can be pretty appalling. You do get to see the worn 1960's-looking "Welcome" sign on the Marina's exterior, but the approach involves walking alongside what looks like an abandoned shipping-container park with a heck of a lot of graffitti.
Once you've reached the brutal-looking concrete perimeter wall (which looks like something from a science fiction film set), you might expect to see some boats ... but instead you'll find yourself looking at a filled-in dock that's been turned into a car park, graced in the distance by a super-sized ASDA. It's not even a particularly nice car park, it didn't seem to occur to the designers to paint the tarmac blue, or to fly pennants from the lampposts, or to do anything to make it look more nautical, and less like part of an industrial estate in Slough. And because it's sunken and surrounded by the sea wall, you might as well be landlocked.
If you trudge across the giant carpark, and pass ASDA, eventually you might find some directions for how to get to see the pretty boats and the other attractions, but it's not a great introduction.
Verdict: if you like boats, and want to see the Marina, take the bus. The bus takes you past all the grotty stuff and straight to the good bits.