As primarily an underwater shooter, I can say for certain that you don't need fast AF, just accurate AF. Also, take note, the BBC Wildlife Photo of the Year was a shot made by Doug Perrine with the Canon D60. If you can't get good images with 8MP, 12.8 won't help much, not to mention the wait you will have for housings to be released for the 5D. There are a lot of housings out for the 20D.
I think you qualified you needs in your opening statement, you're posting them on a website. Web images are typically 6---800 pixel at it's longest dimension. You won't be able to tell between a 6,8 or 12MP shot from that. I shoot with a 10D in a Subal housing. While not a full time photo pro, I've sold both topside and underwater images taken with the 10D. 20x30 blowups of thimble sized pygmy seahorses show amazing detail.
The primary reason for my going to a 5D is the Wide Angle use. My 15mm Fisheye and 16-35L will once again regain their full image circle capability. Now, I can use my 16-35L and still be within reasonable range to say a whale shark or encompass more of the reef scene I'm taking while still allowing me to get close in an light the scene with my strobes.
Compared to flying birds, surfers are relatively slow, many outstanding flight images are taken with 3-5fps cameras. The 5D has the buffer size to capture a good 5 sec (if shooting RAW) run of a surfer or about 13 secs in JPG....you'll likely have a good chance of capturing what you want...the 20D also has a decent buffer size for JPGs.
If you're not making money at it, if it's a financial stretch for you to get a 5D or 1D...keep the 20D and shoot it...LOTS. Housings for the 20D will be subtantially cheaper than the 1D's and still cheaper than the 5D. But, if money's no problem, do what you want!
My 2 cents.
s.