The mood of the poem is sad and dark, describing where the bones of someone's father are laying (at the bottom of the ocean.). Alliteration helps convey this mood because the repetition of sounds, especially the 'f' and 's' sounds, make a sound that resembles the rush of waves or the sound you hear if you're underwater. 
The first example is in the first line, the 'f' sound is repeated in "Full Fathom Five thy Father lies". Then, for a second example, in lines 4-7, the 's' sound is repeated in "Suffer a Sea-change / Into Something rich and Strange. / Sea-nymphS". Again, those two sounds represent the sound and feeling of being underwater where the body is resting. 
 
        
                    
             
        
        
        
B. The observers up front were asked to sit down so that the ones in the back could see. When you are talking about people, you use sit, not set.
        
                    
             
        
        
        
Where is the following statements
        
             
        
        
        
Tituba was one of the first people to be accused of witchcraft in Salem, and she was a black slave and there was a lot of racial discrimination
        
             
        
        
        
It is true that Dr. Samuel Johnson compiled the first dictionary by using the historical method of research.