How difficult is it to eat and drink during ramadan in Thailand and Malaysia?

Having been to both during Ramadan:

In all reasonably touristy areas in Thailand, including the southern resort islands, you basically will not notice Ramadan at all -- pretty much everything is open as usual.

Malaysia, though, is a different story. While you certainly can get drinks and food, most places that stay open do so a little discreetly, with curtains on the windows etc, and you'll want to show respect to people who are fasting by not eating, drinking or smoking in public. The flip side of the coin is that once the time for buka puasa (breaking the fast) rolls around at sunset, many restaurants will be packed with Muslims, so you may actually want to beat the rush and grab your dinner before 6! Out in the nearly 100% Malay countryside, though, virtually everything will shut down during the day.

(Anecdote: I once had the slightly surreal experience of walking into a packed KFC in KL Sentral, getting my dinner and sitting down -- only to realize that nobody in the restaurant was touching their food. So I took the cue, waited for a few minutes until the call to prayer started, and then joined the ravenous horde in tucking into my fried chicken.)

The other travel glitch to beware of is that the end of Ramadan (Eid ul-Fitr, or Hari Raya Puasa in Malay) is the local equivalent of Christmas, when everybody takes time off and goes back to their families. This means most transportation is fully booked for several days and even the traffic jams are legendary. Plan ahead so you can avoid travel at this time.

Thailand is over 80% Theravada Buddhist. I have lived here for years and the average Thai would not even know what Ramadan is - or at least would not think it concerns them in any way. There are pockets of Muslims communities around Thailand.

In CM there is a fair sized community, they will honour Ramadan tradition, but are not at all extreme - so again, even if you stayed in the middle of their community you would not notice much (maybe a lack of food carts that day, but plenty around the corner!)

In the south, especially very far south around Yala, Narathiwat, Songkhla and Satun, the Muslims are known to be more extreme in their views (Malaysian) - but it is very doubtful you will be holidaying anywhere near these areas (if so Ramadan would be the least of your worried - not getting blown to pieces at the local 7-11 would be higher on the list!)