My 4yo is 2yrs post t&a surgeries and snores at nite. Her snores sound like she is breathing through water. What could be obstructing now?

? Allergies. Dr labarbera 's answer is solid. But another possibility for nighttime snoring are nasal allergies. Ask your pediatrician regarding that possibility.
Possible regrowth. There may have been some regrowth of his tonsils or adenoids. Has he had a sleep study performed?
Adenoid tissue. S/p ta - a quick look in the mouth will let you know if the tonsils are the still the problem. A lateral xray of the soft tissue of the neck, will quickly tell if the adenoid tissue has regrown or not enough of it was removed in the first place. Have your doc get the xray if it hasnt been done already before the bronch.
several causes. Adenoids can regrow, though this is not common only 2 years after t;a. Unless the tonsillectomy was a partial one (rarely done) tonsils should never regrow. The most common causes for this are turbinate hypertrophy, allergic rhinitis, lingual tonsillar hypertrophy (another kind of tonsil tissue at the base of tongue) or a small jaw. A pediatric ENT can help evaluate this.
Nasal obstruction. Nasal congestion is a common cause of snoring, and the most common cause of this is allergies. Allergy skin testing is safe, noninvasive, and quick - any child with nasal congestion or snoring should be evaluated by an allergist before non-invasive procedures are performed. I have seen countless adults and children who are highly allergic, but don't come in until some type of surgery hasn't helped.