...Shelley's "Autumn"...

Only published after his death by Mrs. Shelley, Autumn explores a theme common for Percy in the year of 1820, specifically of death and decay.  His friend, Keats, would die within a year of these musings and Percy not much longer after that.  What would these passions foretell.

"The warm sun is failing, the bleak wind is wailing,
The bare boughs are sighing, the pale flowers are dying,

And the year
On the earth, her death-bed, in a shroud of leaves dead,
Is lying."

In Death, published in the same way:

"All things that we love and cherish,

Like ourselves, must fade and perish;

Such is our rude mortal lot - 

Love itself would, did they not."

And, in 1822, a very lyrical piece, Lines, is much more defined in spirit:

"When the lamp is shattered,

The light in the dust lies dead;

When the cloud is scattered,

The rainbow's glory is shed;

When the lute is broken,

Sweet tones are remembered not;

When the lips have spoken,

Love accents are soon forgot."

And, then, the poet's death.  Considering the wealth of permutations in the cause, he did suffer drowning.  But, again in consideration, let's go with the most fantastic of reasons:

Shelley's ship, the Don Juan, was eventually run asunder by the political agents of Wales, who sought to rob him of materials that would have created great upheaval through-out the West.  In the process of fighting a ship of twenty men, Percy would succumb only when he ran out of shot.  At this, he resorted to using hand-to-hand techniques learned by pirates in the western coastal towns around Pisa.  He at last took the material and drowned himself.  For country!

That's what one could call speculation.