Drowning dreams sit closer to physiology than almost any other image, and the honest entry says so first. Sleep medicine documents that disordered breathing in sleep, apnoea above all, can surface in dreams as suffocation, choking and drowning, the labouring body supplying the plot; a recurring drowning dream is one of the few dream patterns for which a medical question, does the sleeper stop breathing, is a genuinely useful first response. No symbolism should be read until that layer has been considered.
Where breath is not the issue, the traditions take over. The old handbooks read drowning toward ruin, being over one's head in the exact sense the idiom preserves, and the folk dictionaries followed with losses, debts and entanglements closing over the dreamer. Psychoanalysis, reading water as the element of the earliest life, took sinking and rescue from water into its account of birth imagery, a reading that belongs to that theory.
The modern composite reads drowning as the extremity of the water dreams: not sadness but submersion, the point where feeling or circumstance stops being something the dreamer is in and becomes something the dreamer is under. Interpreters attend to the dream's ending, rescue, landfall, sinking, or the strange discovery that breathing underwater is possible after all, that last turn being widely reported and generally read as the dream's own answer, the feared element proving survivable.