Amount of Earths water with consideration to space travel?
Everyone always says that there has always been, and always will be, the same amount of water on planet Earth.


Your premise is wrong. The surface and atmosphere of the Earth keeps gaining and losing water all the time. I have no idea where is that coming from, but no scientists say the amount of water is constant.


The Earth's atmosphere is leaking gas to space. This includes mostly hydrogen gas, which is a major component of water, and also some minor amounts of molecular water.
Earth is gaining water (and a bunch of other stuff) from meteorites and dust hitting the Earth. Dust accounts for the majority of the mass flux to the Earth.
The Earth's surface and atmosphere are getting huge amount of water from volcanic eruptions. H2O is the major component of volcanic gas in most volcanic eruptions.
Earth is losing water to the mantle in subduction zones and in lithified sediments.


All of these water mass fluxes are orders of magnitude larger than the amount of water carried to the international space station. It's like asking whether you can deplete a beach from sand by taking single sand grains out of it, when you're gaining and lose huge amounts of sand by ocean wave action.