My guess is POTUS has already been given a prophylactic course of hydroxychloroquine. It has a half-life of ~50 days, so the protective effect should last quite a while.
https://www.ncbi.nlm.nih.gov/pubmed/26246395
This was my thought also. And it made me wonder, when we were watching the water bottles, did that water have a touch of quinine? Tonic water?
President Trump has lived an exceedingly healthy life. His admitted germophobe tendancies may have prompted him to drink a little tonic water daily as a lifestyle.
I don't think "watch the water" was exclusively this, but multiple meanings are there.