I agree abrasives like ajax, comet, and even soft scub will get your boat clean but the first time you step into it, the dirt sticks in all the tiny scratches you just made with the abrasive cleanners. The boat will be filthy again in no time. Usually about ten minutes after you boat the first blue fish.
I bought a 1998 WA that was in good shape but really needed a good cleaning. The boat was covered with stuborn fish gut stains. Tried just about everything you could think of to get it clean. Tried all the abrassive stuff that got it clean but I found myself scrubbing the deck more times than I would like to remember. Finally I decided to try something different.
First, I scrubbed the boat with some good boat soap and power washed as much dirt off as possible. then I used heavy duty compounds made by 3M. I started with their super duty compound and then use a finishing compound called Finesse-It II, also made by 3M. You can compound the deck by hand or buy a buffing machine that has a variable speed control. I used the machine just because of the condition of my deck. Make sure you get the right pads for compound cleanners. Slow speeds work the best, otherwise you will be slinging compound all over your boat. Also make sure you do small areas at a time. After cleanning off the compound, I applied three coats of 3M paste wax. The deck looks brand new and stayed clean a lot longer. Believe me I learned the hard way and there is no substitute for wax. Most dirt and some stains will not stick to the wax making cleanning between outtings a lot easier.
The challege is always going to be how to keep it clean.
Bottom line - clean it good once and wax the hell out of it. This will help it stay clean a lot longer. After each outing try to use cleanners that will not remove the wax and that are not abrasive. Places like West Marine and others sell a number of cleanner/wax products that will help you maintain the deck once you have a few coats of wax down. For me, this method seems to work the best and will keep your deck as clean as possible for as long as possible.
Boat cleaning
If you have some tough spots dillute the ZEP degreaser with 50% water, spray on the spot, agitate with your fingers (little circles) until you get a little foam, let sit for 3-5 minutes and rinse off. Do not let the ZEP dry up! 
