After observing my hives, i noticed that about 10 to 15% of returning bees carry pollen. Is it therefore reasonable to assume that those without pollen are carrying nectar? Are some carrying water or does all bees need to consume water for proper hydration?
They can all carry anything. If they come back without pollen, they may have water, nectar or a mixture of both (dilute nectar), depending on what the hive needs. If they come back with pollen, they may also be carrying nectar or water - that is why some bees miss the landing board and end up on the ground! They didn’t realize how heavy they might be as they slow down to land… 
Human pilots do that too, but it is much messier. At least the bee can rest for a bit, then try again! 
Dawn (private pilot, never crashed…)
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They can carry resin to make propolis too.
If I scrape hives on a sunny autumn day I always have help from a few bees.
There are a lot of bees carrying water on a hot day. There are a lot carrying nectar. Some are carrying pollen. Some propolis. Some have pollen and nectar.
Thanks for all the replies.