According to PlanDisney....
"both smoking areas are found on Deck 14, which should make them easy to remember, as well! One is located on Deck 14 Forward near
Currents, and the other is Deck 14, Aft just above the Quiet Cove Pool, which is accessible by a set of stairs from the Quiet Cove area"
It appears there are some smoking areas on each ship that only allow smoking between 6 p.m. and 6 a.m. and some that are accessible any time. There is no smoking in the staterooms or on verandahs.
The only info I could find for
Castaway Cay is from 2020 so can't guarantee things haven't changed. At that time, PlanDisney said...
"Smoking is also permitted in outdoor areas on
Castaway Cay. If you smoke in common areas on the island, be considerate of those around you and place all litter in the appropriate receptacles, not on the ground."