I know many will hate me saying this but I really did not like 303. It didn't seem to do a good job of protecting like I thought it would. I find that Meguiars All Purpose Cleaner cut 4 to 1 with a vinyl brush cleans vinyl very well. I use Boat Bling's Vinyl conditioner or Meguiars #40 vinyl and rubber cleaner / uv protection.
http://www.meguiars.com/?detailer-engine-degreasers-car-cleaners/All-Purpose-Cleaner Although it does not smell very nice Hot Sauce by Boat Bling is very good when you pull your boat from the water as an in between quick detailer. I did just polish my boat using their Pro Polish and I was very pleased with the product overall.
I think Zaino is the way to go if you are going to do a good cleaning / wax on the boat but for much cheaper the Meguiar's Flagship is also very good. Driven also makes a very good marine polish as well.