Saigon heat dehydrates faster than you'd think. Aim for 3+ liters of water per day, more if you're walking.
Coconut water (nước dừa) is the local electrolyte drink — refreshing and natural. Carts cost 20,000–30,000 VND for a whole young coconut.
Avoid drinking only sweet drinks (sugarcane juice, milk tea) — they accelerate dehydration in the heat.