So here is a whack-a-doodle idea maybe. Since your in the stands, I don't think a 400 2.8 is really all that practical, for you or the people around you. I have a Mk II of that lens, love it, but for indoor tightly packed venues, not ideal. The 300 is much more manageable, and cheaper. You can also marry it with a good crop sensor camera that gives you a lot more flexibility. I used to shoot at Duke with a 7D and an old Sigma 120-300, and the range was perfect. I had that and a 1D with a 70-200 mounted and with the two I had the venue covered.
Never shot the 200-400, so can't speak to that one at all. If you are shooting from seats at the end of the deck Im thinking 200 is still going to be a bit long. As mentioned before a two body 70-200 and a 200-400 would cover a lot of options. But I wouldn't personally go with the prime 400 unless you can get a really high and steep angle at the water.
The other thing is with big glass you need to show up early to let the lens acclimate to the venue. I mistakenly didn't give myself enough time once and spent the first 30 minutes defogger the dang thing between each heat. Sounds like you've done this a bit, so that isn't news to you.