Distilled water belongs in a steam iron, not in a live snake. As I said, snakes (& other living creatures) need the trace minerals (electrolytes) found in normal water.
I think some people* may be confusing other bottled water (spring water etc) with distilled water...they are misinformed. Also, tap water varies from one place to another:
if there is a lot of chlorination in your water (or other chemicals to make it safe to drink), you might want to boil it first so the chlorine will off-gas. Just don't use distilled.
*I think they 'meant well', maybe they were worried about how your local water is purified, & that the chemicals are bad for snakes, which is true. Another thing you can
do if this is the case is to get a water treatment product from a tropical fish or pet store that removes the chlorine/chloramine & heavy metals in tap water. 2 products
that I know of are Instant Ocean brand "Marine Conditioner" ("makes tap water safe" for fish) & Reptisafe "Instant terrarium water conditioner" for all snakes, turtles, etc.
Bigger tanks are great for SOME kinds of snakes...but that's NOT the kind you've got.You have a BP & they like to live in tight rock caves in the wild. So much for that
"expert"...(& yes, if you bought from a pet store, they were either uninformed- very common!- or trying to make a bigger sale! sadly, that's also common)
How big is your snake? (length, age?)