With out getting the obvious reply, what happens when they capsize? Do they have something to hold it in place?
Nothing to hold in, so you lose it. But there were hardly any capsizes at the worlds anyway.
I keep my water bottle in the rear beam with a bungee cord holding the cap. Rather easy and unobtrusive position. And use the diamonds as a place to use the first bottle I will consume on the way to the start, so by the time the race has started (or about to) it is empty and thrown into a rescue boat.
