Opposite to “A rising tide lifts all boats”

On a similarly nautical theme "They're all in the same boat" could be worked into a sentence to get something close. This phrase implies that they have a shared fate; usually with negative connotations.

The opposite of the above statement is that a falling tide sinks all ships. As per DJClayworth in the comments.

Beyond this, I fail to see the fascination with trying to connect one idiom to another. Aren't idioms supposed to be illogical, non-literal and unconnected bits of speech? Unique bits (with their own unique opposites).

Wolfgang Mieder, A Dictionary of American Proverbs (1992) suggests three (seemingly related) sayings that use rain as a metaphor. The most familiar of these (noted by Greybeard in a comment beneath the posted question) is

Rain falls alike on the just and the unjust.

The sense here is that when large-scale trouble occurs, there is no discrimination between victims on the basis of their personal virtue or vice.

A similar sentiment is expressed in this alternative saying:

All who travel in the rain get wet.

...except that here the question of personal merit doesn't come up at all.

A slightly different take on the underlying idea appears in this saying:

The rain that rains on everybody else can't keep you dry.

That is, the particular misfortunes that others suffer in a general calamity don't immunize you from injury or even (in any practical sense) reduce your own exposure to harm.