Twenty-eight young men bathe by the shore,
Twenty-eight young men and all so friendly;
Twenty-eight years of womanly life and all so lonesome. --This refers to the fact that she is an older woman, has had 28 years of "womanly life" (on top of her non-womanly life as a girl, one supposes.) Probably early 40s? There are also 28 men bathing by the shore she lives on (men would have bathed in the nude, most likely, around this time.)
She owns the fine house by the rise of the bank,
She hides handsome and richly drest aft the blinds of the window. --She is respectable and pretty, from a wealthy family or with a wealthy husband. She keeps the blinds drawn to reduce the chance of her being seen by the bathers.
Which of the young men does she like the best?
Ah the homeliest of them is beautiful to her. --She has been without male companionship...the least attractive of the men is still enough to get her motor running.
Where are you off to, lady? for I see you,
You splash in the water there, yet stay stock still in your room. --Does one of the bathers see her up there in the window? Maybe. In her mind she is splashing in the water with the men, but in reality she is perfectly still up in her room. A stark contrast between her reality and her desires.
Dancing and laughing along the beach came the twenty-ninth bather,
The rest did not see her, but she saw them and loved them. --Perhaps the 29th bather is the one who is able to see her? She is enamored with all of them, the sexuality and freedom they represent.
The beards of the young men glisten’d with wet, it ran from their long hair,
Little streams pass’d all over their bodies. --Erotic imagery...the types of things she is noticing. The reverse of a Sports Illustrated swimsuit issue.
An unseen hand also pass’d over their bodies,
It descended tremblingly from their temples and ribs. --She's fantasizing about touching them.
The young men float on their backs, their white bellies bulge to the
sun, they do not ask who seizes fast to them,
They do not know who puffs and declines with pendant and bending arch,
They do not think whom they souse with spray." --The men are unaware of the effect that hey have on this woman because all (except perhaps one) is unaware that she is watching. I believe in this section she is fantasizing about having sex with one of them.