The Great Gatsby, the green Sound, stagnant in the heat

I'd like to know if, in the chapter Seven of The Great Gatsby, the word "stagnant" is used in its original meaning, "not flowing or running" and so with the metaphorical connotations of "foul and stale", or if it is used in a milder sense, just to suggest immobility:

I went with them out to the veranda. On the green Sound, stagnant in the heat, one small sail crawled slowly toward the fresher sea.

Thank you.