My, you do rattle on, don't you? If you read the description it is obvious the schooner was running alongside on a parallel, but opposite, heading to the fatboat. While you could technically fire a broadside at the stern of a retreating vessel, it would not be very efficient.

As to the use of "turgid sea", I think you will find plenty of precedent for the phrase, which has been used among others by such luminaries as Matthew Arnold and Edgar Allen Poe:

About noon, as nearly as we could guess, our attention was again arrested by the appearance of the sun. It gave out no light, properly so called, but a dull and sullen glow without reflection, as if all its rays were polarized. Just before sinking within the turgid sea, its central fires suddenly went out, as if hurriedly extinguished by some unaccountable power.

So I find there is adequate usage established to justify the phrase, even in the face of your disapprobation, Sirrah.

A