The audience does not hear the first part of the conversation between Beatrice and Benedick, which apparently begins when they are in the back of the stage. But from the first part of the spoken dialogue, the audience learns that Benedick (in disguise) has told Beatrice that he has heard rumors about her. Beatrice quickly asserts that the rumors must have come from Benedick. The disguised Benedick claims that he does not know anyone by that name.
Beatrice then takes the opportunity to describe Benedick to her dance partner. She describes him as the "Prince's jester" and as a "very dull fool" (line 118). Beatrice then adds that Benedick is not witty but only rude in his speech; and other men only laugh at him, not with him. She is saying that he is an object of ridicule.
Beatrice concludes this description with the following:
I am sure he is in the fleet. I would he had boarded me. (122-23)
Beatrice here uses witty metaphors. In the navy a fleet is a group of warships under one commander. The word boarded is used when a sailor goes onto or on board a ship (or can even be used when a group of pirates attacks another ship). In this instance, then, Beatrice is stating that she wished (the word would means wish here) that Benedick had attempted to dance with her or even tried to attack her verbally. Beatrice is claiming that she would easily counter his verbal attack and put him in his place. She is claiming that she could easily beat him in a battle of wits. The Renaissance audience, ever ready for a double-entendre (a pun that includes a sexual reference or joke), would also know, however, that boarded has yet another meaning. It also suggests a man going on top of a woman for sexual intercourse. Subconsciously, perhaps, Beatrice may also be revealing her desire for Benedick.
The short conversation ends with Beatrice telling her dance partner that if he ever does meet up with Beatrice and tells him what she had said, then he will tell a few bad jokes ("comparisons") about her that no one will laugh at and that then he will get angry and not eat his supper (lines 126-29). Benedick, needless to say, is not at all happy about hearing Beatrice talk about him in this manner.