What do the words "canker blossom" and "waterfly" mean?

William Shakespeare is well known for his extensive use of metaphors, similes, symbolism and imagery in his writing. Although understanding his works requires knowledge of the English terms used at the time, it also requires reading the terms in their context.

____________________________

"Canker blossom" is mentioned in Act 3 Scene 2 of A Midsummer Night's Dream in this context:

HERMIA (to HELENA):

You juggler! You canker-blossom!

You thief of love! What, have you come by night

And stol'n my love’s heart from him?

"Canker" is technically a reference to a cankerworm which eats through the blossoms of plants. In colloquial use though, it may refer to anything which destroys or causes bad things to occur.

"Blossom" is obviously the blossom of plants, but if you read the following lines, we can infer that "blossom" refers to the "blossom of love." 

So Hermia is calling Helena a destroyer of blossoming love. "Have you come by night and stol'n my love's heart from him?" They both love the same man but Helena has won him over and literally destroyed the blossoming love of Hermia.

____________________________

"Waterfly" is mentioned in Act 5 Scene 2 of Hamlet in this context:

HORATIO

Peace.—Who comes here?

Enter young OSRIC, a courtier, hat in hand

OSRIC

Your lordship is right welcome back to Denmark.

HAMLET

I humbly thank you, sir. 

(aside to HORATIO) Dost know this water-fly?

HORATIO

(aside to HAMLET) No, my good lord.

HAMLET

(aside to HORATIO) Thy state is the more gracious, for ’tis a vice to know him. He hath much land, and fertile. Let a beast be lord of beasts and his crib shall stand at the king’s mess. 'Tis a chough, but, as I say, spacious in the possession of dirt.

A "waterfly" is an insect that aimlessly hovers over water. Hamlet is not very fond of Osric (who has just walked in) and tells Horatio that he is lucky not to know him. He thinks of Osric as an aimless, annoying person. Hamlet says that he is a "chough" which is thought to be a type of loud bird, so he is referring to someone who talks a lot, who is loud and crude but who receives respect because he is wealthy and owns a lot of land. So "waterfly" is an insult in all senses of the word.