If I stopped for a pub lunch I would ask the bar man for a bucket of water for the horses. Otherwise not sure but TBH i've hunted for that long and I wouldn't stop to offer a horse water then.
I used to work at a riding holiday centre where we did long (as in 4-5 hour) rides, stopping for lunch on the way. Horses didnt often get water during the day poor things were always dragging clients across the yard to the trough when we got back. Some rides went through rivers so that was ok, and one stopped at a lake for lunch but mostly they went without
We take ours to work (giving carriage rides) and they can work from 10am - 5pm. We always take water and a bucket and often offer them drinks, but they rarely have any. Therefore, I wouldn't worry too much. Pubs around here will often find a bowl or bucket and fill it for the horses.
I think you'll find places - puddles or streams - that if they're desperate for a drink, they'lll take advantage of.
But I've taken Twiglet on a 22 mile, 7 hour ride, and he refused to drink a drop! On a hot day too.......silly horse. Our hacking partner was drinking from the water jump on the XC course, but Twiglet refused until he got back to the field.