Is there an English equivalent of the proverb 水清ければ魚棲まず (if the water is clear, fish won't live there)?

I cannot think of any particular English saying or proverb which encapsulates the idea of not wanting to work with someone who is irreproachable. In part this is because I see some of traits you ascribe to such a person are orthagonal to one another; for instance, being sincere is different from being sententious, and being scrupulous is different from being sanctimonious, and having one trait does not imply having the other.

That said, the general sense that someone can be "too good" is well-understood in the Anglophone world, and there are many expressions for such people themselves, if not for interacting with them. Sometimes, hesitation to interact with such people can arise from feelings of inadequacy or unwillingness to match their standard, but more often it is the suspicion that such people are simple and therefore naive, self-righteous and therefore hypocritical, or insincere and therefore generally dubious.

For example, someone might be compared to an archetype or famous example of a good citizen or humanitarian either to disparage or to praise. One may also deny being such a person to demonstrate humility.

• Parker, you are such a boy scout.
• I don't want to be seen as some sort of Mother Teresa.
• A contemporary Albert Schweitzer type.
• A real Ned Flanders kind of word.
• Carter was, comparatively speaking, a regular Audie Murphy.

Other answers have already pointed out several terms for people who are mainly interested in being perceived as good, or at least perceived as morally superior to you, but I do not think that is what your question is asking about. Someone who is ostentatiously well-behaved may be a goody two-shoes or a goody-goody, informally. If they like to compare their own behavior favorably to yours, they are acting holier-than-thou, or on a high horse.

I can’t think of a universally used proverb, but more a collection of sayings which somehow criticise the moral high ground.

The "moral high ground" is the equivalent of your "clear water". Someone who occupies the moral high ground is often seen as a "goody two-shoes", but the moral high ground is also described as "cold" or "lonely".

Examples include:

• Does it ever get cold on the moral high ground?

• Your moral high ground is so elevated I’m surprised you have any oxygen left

It does not exactly translate to "if the water is clear, fish won't live there" but an idiom which works for the situation is: get off (one's) high horse

As in:

Get off your high horse or you'll not make any friends!

TFD(idioms):

get off (one's) high horse
To stop acting as if one is better than other people; to stop being arrogant or haughty.

Sam is never going to make friends here until he gets off his high horse and stops acting like he knows more than all of us.

Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.