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.
While the Moon shall be South descending in Scorpio or Pisces, begin not then to build; for a fabrick then erected quickly comes to ruin. - William Lilly (1602.-1681.)