Drowning in a dream typically reflects being emotionally overwhelmed -- more demands, feelings, or responsibilities than you feel able to keep your head above. The specific moment you struggle for air can point to exactly where the overwhelm is coming from, whether it's work, a relationship, or an internal pressure you've placed on yourself.
You can address Mercury on the empty signs while the Moon can be addressed on both positions indistinctively. - Picatrix (Andalusia, ~1000.AD)