Several of my dogs, as they got older, became more sensitive to the heat (even though we have AC) and started to sometimes skip breakfast. I learned to not worry about it as long as they ate later in the day and seemed otherwise healthy.
Sometimes adding low sodium chicken broth to the water will encourage them to drink, also I have heard of dogs that like ice cubes in their water when it is hot although none of mine have. You can also add water, broth, tuna can water, goats milk, something like that if they like it to their regular food. That will help keep them hydrated.