There is no shortage of options for triathletes in terms of how to carry liquids on the bike. There are rear cage holders, aero bottles, aerobar-mounted bottles, aerobar-mounted cages, frame-mounted straw systems, saddle-mounted straw systems, and of course, the standard frame bottle cage. That's a big list. So what's the best way to go?
The balanced answer would be to evaluate different people's needs, and evaluating the merits of different systems in different scenarios. But my gut can't let me do that. Well, not completely. Ideal storage will indeed depend on distance (a 5-mile ride won't require any, while a 112-mile ride will need a lot). But I have some pretty solidified opinions about hydration storage, and I'm gonna give 'em to you. In the end, that's why you're reading this anyway, right?
The short version is - you don't need anything fancy. Sure, there are some pretty slick aftermarket solutions available. But there are also some pretty ugly, clunky, heavy ones. Some of my favorite setups don't require anything except a couple standard bottles and some zip ties.
The very first stop on our hydration tour is the aerobars. But forget those ugly aero bottles with their ridiculous mounts, and unavoidable splashes. You want to mount a standard cage between those bars. I call it the Between-The-Arms (BTA) bottle, and its benefits are many fold. First, this is the most aerodynamic way to mount any bottle according to several industry experts. Take a look at the picture adjacent - there's a bottle in there, even though you can't see it. It's totally hidden from the wind. Mark Cote, the aerodynamicist for Specialized, has even said it tests faster than not having a bottle in there at all - because it fills up the gap between your arms and the wind flows around them better.
Moreover, the BTA bottle is quite versatile. For short rides and races, I use a special bottle I've rigged up, drilled out for a straw, which I glued in permanently. This bottle is the easiest thing in the world to drink out of - just dip your head and drink. But it doesn't EVER splash, and if I tilt the straw to the right, it rests right up against my arm, not contributing any additional frontal area. For longer rides, or during a longer race, I can toss a bottle out and take on a course bottle. This is exactly what Chris McCormack did at the 2010 Ironman World Championships on his way to victory.
The best thing about this solution is how EASY it is to implement. Don't buy any of those "torpedo" mounts. Seriously, just save your money. All you need to mount the cage are a couple of zip ties. These are dirt cheap, only a couple of cents each. I buy mine in a 100-pack at Home Depot, so that I can experiment with different setups. You want to lock your cage in place, and doing so might require you to drill a couple holes in your bottle cage. But don't be afraid - cages are cheap too, so go nuts with the power drill.
Inviscid Design has come out with a great riff on this idea called the Speedfil A2. Read my review of that product for more specifics. Basically, it's a module that replaces the screw-on top of a standard bottle. It's easy to refill on the fly, and has a built-in straw. Because it's a little more complex than just a standard bottle, it takes a little more care to set it up in the optimal position. But if you take the time to do so, it's a great system (and the one that's currently sitting on my bike for all my rides). Another AWESOME product of this ilk is the Profile Design HC Bottle, which is currently my go-to BTA product.
Finally, for a very good alternative to the BTA bottle, check out the TorHans products. Usually, I go BTA, but TorHans puts a lot of very good thought into their setups, and they're worth a look. It's more of a turnkey product, if you can't be bothered to mess with perfecting a BTA setup.
If you need more than one bottle - particularly on half-ironman courses and longer, you're likely going to be using it for a concentrated solution with a bunch of calories in it. Many top athletes like this solution. With your swappable BTA bottle up front for water or course drinks, you're best off using the downtube (or seat tube) bottle for a good 1,000 calorie or so mix. You take small sips from this every now and again, and a steady stream from your BTA bottle, which is easier to get to. Since you won't be swapping this bottle, it makes sense to make it as aero as possible, and that means one of several choices. Specialized, Arundel, Bontrager, and Profile Design all make good bottles, some of them integrate better with certain frames than others. If you have a Specialized bike, get the Specialized bottle and mount it on the downtube. If you have a Trek, get the Bontrager and mount it on the seat tube (it fits better there). The general rule is to put it where it better "completes" the shape of your bike without leaving gaps.
For longer training rides, you can easily swap to a standard cage instead. But I don't recommend leaving them there on race day. There are other good places to put round bottles (between your bars and behind the saddle), you might want to consider other options than round bottles on the frame. On the other hand, Gerard Vroomen of Cervelo has publicly stated that in some cases, a standard bottle on the downtube can have a negligible aero penalty. But more recent data on modern frames (even by Cervelo) suggests that round bottles aren't the best option for frame hydration.
For the two applications above, cage selection isn't critical - any decent cage will keep your bottles in place, because you aren't putting them on a rear-mounted bottle launcher. But if you really want a rear bottle, mount up a single one Chris Lieto style, right behind the saddle. You can do it with zip ties for the el cheapo solution (that's how Chris does it). But with a bottle behind the saddle, you'll want a good cage. The best cages have a solid lip that fits into the bottle's choke point. For bling factor, I like Arundel's Trident cages. They're pricey, but I've had a pair of them for six years, and they're still going strong. They've followed me from bike to bike. On the cheap, get a Specialized Rib Cage. They're fantastic. And if you're a Dash Saddles acolyte, they make the very snazzy TT9 with integrated cage bosses so you can skip the zip ties.
Thing you need more? Reconsider
Think about this: Chris McCormack is a big guy - according to him, he's the heaviest man ever to win the Ironman World Championships. He has a harder time with the heat in Kona, and you'd think he'd want more bottles than anyone. But on the way to his 2010 victory, Macca was fine with just two bottles on his bike. One BTA bottle, and one round cage on his downtube. Both of them were standard cages, and the BTA cage was zip-tied on there, exactly like I'm recommending here. Some people argue that they do long, unsupported rides, and need at least four bottles to make it through. Okay, no problem. Just plan your ride so that you can stop at a gas station at the halfway point. It'll take you less than two minutes to stop, fill up your two empty bottles, and hop back on. I recently had the chance to watch several of the top athletes in the sport, including two-time World Champion Craig Alexander, go on a 112-mile unsupported training ride. No one had more than two bottle cages anywhere. Two hours into their ride, they did exactly what I've recommended here. They stopped and refilled. For the weight weenies among you (and I'll admit, I'm one of them), carrying four bottles is a big no-no. That's close to eight pounds you're adding to your bike. Almost 3,500 grams of extra weight!
Forget the enormous, clunky, heavy, and god-awful-hideous contraptions out there. The most important thing I can tell you is to get your BTA bottle set up right. It's all you'll need most of the time. Pop a straw in it for the extreme convenience, if you want (I love mine). A downtube bottle, be it aero or standard, will get you through the longer rides. If you're doing ultra rides where you absolutely, positively must have more liquid on board, get a Camelbak. Really. But the rest of the time, the guide above should work. And that really is the long and the short of it.