In the US they are ruthless about water bottles. I've never had a carryon weight on a US flight and have never seen one rejected. No matter how much over sized. The only times I've seen people made to check a carryon is when it wouldn't fit.
how about in terms of # + size of carry-on? ie taking a medium-size laptop+lens bag along with a big backpack for housing.
i havent taken a US domestic flight in some time, and just thinking ahead for JFK-LAX flight i'll be taking for a trip later this year on AA (economy and booked as one-way separately).
For HKG: note the critical thing is the bag size right before you go through security/immigration. I used to carry a slightly "taller" backpack and got sent back to the counter (where it was weighed and I had to make noise) all the time before getting a smaller one. (no issues with CX, Air Pacific, MAS, SQ, Cebu Pacific even). I am hearing that Phillipine Air routinely checks carry on recently though.
Intra-asia Ive also had no issues in Singapore, KL, Male, Jakarta, Manila. MAS also quite generous for economy checkin so long as you say "diving equipment" (total 32kg))