It's nice to have a handy axiom ("Needs to be food grade") in case a question arises about leaching from plastics, but I don't think it's in one's interest to depend on these alone to make decisions that affect one's health.
A lot of potentially deleterious things can diffuse out of a plastic into a food or beverage, and some plastics are more of a risk than others. Some plastics that are benign at room temperature and neutral pH can leach xenoestrogens or mutagens if exposed to high temperatures or extremes of pH.
The concern about contamination of food and water from plastics comes from problems associated with polycarbonates, polyvinyl chloride, and to a lesser degree, polyethylene terephthalate ("polyester") and epoxies. Bisphenol-A and phthalates are the principal contaminants of interest, although other compounds are coming under more scrutiny.
Nalgene bottles, which are food-grade, are polycarbonate and have been shown to leach BPA. Many conventional bladders for hydration packs are food-grade PVC, but leach both BPA and phthalates. Aluminum beverage cans are lined with food-grade epoxy which leaches BPA. In Japan, aluminum beverage cans are now lined with polyester due to leaching from epoxies. Polyester is thought to be the safest of the questionable plastics, but food-grade polyester can leach antimony, phosphites, and acetaldehyde (which has a bad taste but isn't very toxic).
Polyurethane is not considered one of the questionable plastics. Many companies that make hydration bladders have switched from PVC to TPU (thermoplastic urethane) due to concerns about leaching from PVC and the better mechanical properties of TPU. Most manufacturers of medical plastic containers and tubing have also switched from PVC to TPU. Heat sealable nylon is lined with TPU.
Even if there are as-yet unrecognized contaminants leaching out of TPU containers, a water bag is made to contain cold water at neutral pH, not something hot, acidic, or alcoholic that would tend to dissolve hazardous solutes in the plastic.
From a pretty basic review of current literature, I think it can be guessed that the TPU lining of heat-sealable nylon, "food grade" or not, is likely to be safer than a polycarbonate Nalgene water bottle, an aluminum soda can, or any of the widely available PVC hydration bladders.