Symbolism, Imagery, Allegory
Water, water everywhere and not a drop to drink. To be fair: Jake Barnes & Co. are way more interested in slurping down anything and everything with alcohol content than messing around with boring ol' water. But still, H2O comes on the scene in a meaningful way.
Water appears on multiple occasions as a symbol of purification and relief. On Jake and Bill’s fishing trip, water seems to have the therapeutic effect of soothing Jake's troubled soul. While the men drink loads of wine while fishing, they first chill it in the river. This seems not only to cool the wine’s temperature but its effect; rather than creating a sense of drunken chaos, the wine rejuvenates them and stimulates Bill’s creativity (which he expresses verbally at a breakneck pace).
When Jake leaves Pamplona for San Sebastian, he wants nothing more than to swim in the ocean. The water relieves and strengthens him, and he feels buoyant and supported:
I undressed in one of the bath-cabins, crossed the narrow line of beach and went into the water. I swam out, trying to swim through the rollers, but having to dive sometimes. Then in the quiet water I turned and floated. Floating I saw only the sky and felt the drop and lift of the swells [...] The water was buoyant and cold. It felt as though you could never sink. (19.28)
Poor Jake finds everything he needs (albeit briefly) in his seaside jaunt. He finds "quiet," he doesn't have to see anything horrible—like war or sadness—and instead "[sees] only the sky," and he gets the feeling of never being able to sink. Compare that last tidbit with what Jake feels most days: a constant sinking sensation.
Finally, Brett is always going off to bathe, signifying her own innate desire to purify herself and perhaps disassociate herself from her (scandalous, for that day and age) actions.