I've read that UCI regulates the dimensions of water bottles to stop riders from adding aerodynamic bits of plastic and calling them "water bottles". What sort of "water bottles" have been used in competition for aerodynamic benefit?
Article 1.3.024 BIS states:
“Bottles shall not be integrated to the frame and may only be located on the down and seat tubes on the inside of the frame. The dimensions of the cross sections of a bottle used in competition must not exceed 10 cm or be less than 4 cm and their capacity must be a minimum of 400 ml and a maximum of 800 ml.”
I think this mainly bans integrated designs such as this BMC Teammachine shown below, or the Specialized Shiv from being used in UCI races. For the latter, in triathlons, no such limitation exists, so you can see it in various non-UCI events and for the BMC, the plastic module (called the "Aero Module" ) can be detached and traditional bottle cages mounted in place for UCI rule compliance.
Other manufacturers also have integrated structures on their dedicated triathlon frames, so that's a common theme where such regulations don't exist.
I should add that these modules are typically optional and there are often dedicated TT/triathlon versions of the same bike, for example there is a Shiv TT which is used in UCI races, it probably doesn't even have mounts for the optional water containment, there is no use for it, simply because it adds more volume/weight than needed on a typical TT race which is under one hour and may even include feeding zones.