Hydrating's not a drag.
If Profile Design spec'd the larger FC35 for 140.6-mile courses, then the smaller FC25 Drink System would be your 70.3 model. It puts everything you need for hydration and caloric-intake needs between your aerobars but shaves off ten ounces of volume for a more compact profile that reduces your cockpit's drag profile.
The water reservoir's straw can be switched to a fore or aft position, depending on the particulars of your aero bar reach and stack. Though it's not as large as the FC35, the FC25 does feature the same splash-back guard, so you can refill it quickly without breaking pace from bottles mounted on your down tube or under your saddle.
The rear compartment of the FC25 System provides catch-all calorie space for anything from trail mix to gels, and the flexible bands criss-crossing the opening help organize and secure your calories. The provided head unit mount means you'll be in complete control when it comes to monitoring those calories as you convert them into watts.