How long is your story?First thoughts: Strangely, I don't find grammar to be the problem in these 13 lines so much as I do mood. If this is a psychological thriller, and assuming there is something nefarious going on in these 13 lines, you're sabotaging the mood with words like "milky," "gentle," "scattered raindrops," as well as with the sentence structure to which Rahl referred.
Not that the sentences are incorrect--alone and at first glance, each seems okay--but together, their cadence, their flow, whatever you want to call it, is too--I don't want to say flowery--maybe poetic. Not nefarious enough.
"Raindrops"--scattered (?) or not--also seems more poetic, softer, as opposed to "rain," which sounds harder, more malevolent for your purposes. (Again, assuming there is something nefarious going on here.)
True, you can use such words as a foil, but I don't get that sense here.
"Carcasses" is good, but it may not be accurate, since "shells" don't have carcasses, and I don't think they can even be considered carcasses, stricly speaking. (I don't eat oysters, so maybe any oyster connoisseurs can set me straight. 
) But I do love the oyster shells. They're different, unexpected, and caught my attention.
Not that this is perfect, but here's something for a quick comparison/suggestion:
The van splashed through murky puddles of rainwater where oyster shells cracked and popped under the black sidewall tires. As the vehicle rolled to a stop, the man inside backhanded the sweat from his forehead and wiped his hand on his trousers. Rain fell as he rolled down his window. He tugged at his leather cap, centering its visor over his eyes, and then leaned back and started—methodically—to pick the wrapping from a new roll of duct tape.
Finally, as Rahl said, a personal note from an editor means you're doing something right. 
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