First of all, it's usually very hard to find a "real origin" for a common saying (unless it was coined by someone in writing). Common sayings constantly develop, change their meaining, become corrupted, etc.
Now for another set of speculations (just so you know, I'm reciting Русская Речь from memory — it's not my own speculation).
Возить воду is a common metaphor for hard, thankless labor, and на нём можно воду возить is said of a physically strong, but meek and servile person who won't protest to this type of toil. This doesn't correspond to any of meanings of сердитый: angry, grumpy, irate.
Dahl mentions this as на сердитых воду возят на упрямой лошади. This can be perceived in two ways:
- An angry person would be forced to drive a stubborn horse to deliver water
- The water for an angry person would be delivered on a stubborn horse. The word сердитый is in the accusative, not prepositive (compare to еду за водой, на тебя привезти?)
The second reading makes much more sense to me: if you are so angry, we'll use a stubborn horse to deliver your water, and the wait will allow you to cool down.
However, the second portion was later lost, and the meaning of the proverb changed. It became: if there's an overabundance of something in your character/behavior, you'll be forced to deliver water.
This "overabundance" may be сердитый (too angry), обиженный (too resentful), добрый (too kind) etc., and all these variants are mentioned in the corpus. Only the last meaning makes sense, and it seems to be the latest reinterpretation.
There is a legend in St. Petersburg that angry water carriers who treated their horses badly, were forced to harness themselves into the water carts and feel on their skin what the poor animals feel.
Here's a photo of the monument to such a carrier I took near the St. Petersburg Water Museum in 2010:

Update:
The actual entry in Dahl reads as
На сердитыхъ воду возятъ, на упрямой лошади.
This, I believe, invalidates the second reading, and the second part doesn't seem to be a part of the saying at all.
Dahl further provides another meaning for сердитый:
сердито взялся за работу, упорно, съ рвенiемъ (пск. твер. Опд.)
Dahl's reading, I believe, is that an industriuos, hard-working horse would be used to carry water, which might be a warning against being too much of an eager beaver.