Does skipping water during exercise make you weaker?

Short answer

No. No study found that skipping water during about an hour of exercise made people weaker. Strength dropped when people lost more than 2-3% of body weight in water, which happened when they started already dehydrated or exercised long in the heat.

Does skipping water during exercise make you weaker?
'No water → dehydration → less strength' — does this formula hold?
A common rule says you must keep sipping water before you feel thirsty, or your strength drops. Others say an hour or two without water is fine. We checked the research on how much body water you have to lose before strength falls.
Losing a lot of body water does cut strength
Strength down 5.5%, muscle endurance down 8.3%
An analysis that pooled 28 papers testing people while dehydrated found strength fell 5.5%, muscle endurance 8.3% and explosive power 5.8%. Jump height did not drop. Dehydrating people with exercise caused bigger drops than sweating it out at rest.
In the weight room, reps dropped
After losing 3% of body weight, about 15% fewer reps
10 people sat in hot water to lose about 3% of body weight, then did 3 sets of 6 lifts. Total reps fell from 169 to 144, about 15% fewer. In a study that dehydrated 7 people by 2.4% and 4.8%, peak force and jumps were the same, but performance in the later squat sets dropped.
But these people were dried out before exercise
During weight training, people drank more than they sweated
Both studies above dehydrated people with heat before the workout. What about real training? 26 people drank 196% of the fluid they sweated out during weight training, while 20 doing aerobic training drank only 56%.
Holding off on water during exercise? 1 hour was fine
In 1 hour of hard cycling, not drinking was 2.5% better
An analysis that pooled 9 cycling studies found that in 1 hour of hard riding, the drinking group was actually 2.5% slower. In 1-2 hours at moderate effort, drinking was 2.1% better, and 3.2% better beyond 2 hours. An analysis of outdoor cycling studies found performance held even with up to 4% of body weight lost.
Heat and long sessions change the story
2 hours at 35°C, 2.9% lost, about 6% weaker
7 people rode for 2 hours at 35°C, then did a 5 km time trial. With thirst matched and less water secretly replaced, they lost 2.9% of body weight and their power fell from 295 to 276 watts, about 6% lower. An analysis of 64 trials and 643 people also found drinking helped more in heat and longer exercise.
How you start matters more
Over 80% showed up already dehydrated
In the 26-person study above, more than 80% started training already dehydrated. When 11 people cut fluids for a day and then exercised in the heat, how fast they could produce force dropped and their landing form got worse.
Bottom line: an hour of exercise won't sap your strength for lack of water
Strength fell after losing more than 2-3% in water
Strength fell when people lost more than 2-3% of body weight in water, and that came from starting dehydrated or exercising long in the heat. For a 70 kg (154 lb) person that is 1.4-2.1 kg. If you start well hydrated, drinking when thirsty during about an hour of exercise is enough.