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I've done burst shots on the 5D2, and it was pretty darn effective if I say so myself. <4FPS doesn't seem like it's a lot, but in your case with person after person jumping into a pool, you get good at it, and if you keep at it throughout the day, you'll get a surprising number of keepers.
I've done this with the standard "everyone jump in the air" shot, "people on water slides," "water balloon fights," and various other forms of action. Other advantages of doing it that way is you can control the shutter speed. For action, that is CRITICAL. If you want to get people frozen in air, you CANNOT do that with video, as they will be motion blurred. You'd have to shoot at a higher fps than 30, which the 5D2 is not capable of. Lastly, video will be limited to 1080p, I suppose it's debatable whether or not you need the full resolution of 5D2, but I'd rather have full resolution than not, as hey, HD space is relatively cheap.