In sailplanes we add up to around 200 litres of water to wing tanks in single-seat gliders (empty weight typically around 280kg) to improve the high speed performance (typically up to 270 km/h these days) and ability to fly comfortably in turbulence.

It's surprising what you can fly formation with, even without an engine [1] :-)

https://www.youtube.com/watch?v=G0icOICQLTc

Dumping (most of the) water before landing:

https://www.youtube.com/watch?v=I4Yv-V7eozk

[1] this particular glider does have a small engine for takeoff, but maximum speed with the engine extended is 180 km/h and here they are flying at 280 km/h.