In nature, Hamsters are drinking dew drops if they´re thirsty, using rivers or seas is not common;
basically cause in their nature environment there arent any. Mostly they get the needed fluids out of veggies, fruits and insects.
In heated rooms the hydration-level of the air is often too low and the temperature is too high. If they are kept in plastic-cages, aquariums or tanks with a water-absorbing bedding, the hydration-level of the air is much less than under normal conditions in nature.
Please have in mind, that hamsters are nocturnal and normally active during the night. In nature this would be the coolest part of the day with probably a fresh and cooling wind. You can´t give your hamster these environmental conditions!
Even if you´re feeding fresh veggies, fruits and insects on a daily basis, your hamster needs additional water ! And if not, what are you going to lose if your hamster isn´t drinking the water ? Nothing. But IF he´s thirsty, there will be water available - and that´s the major point why you should provide water for your hamster.
The dumbest "argument" i´ve ever heard is: water provided in water-bottles is causing diarrhea.
Under very specific conditions a hamster can get diarrhea from drinking water:
If he couldn´t get enough fluids over a week (or longer), the environment is hot and dry and you give him cold water to drink. This will probably cause diarrhea. But keeping a hamster under this conditions is cruel and irresponsible!
Under normal conditions, hamsters will only drink water, if they couldn´t get enough fluids from veggies and fruits. If you´re feeding veggies and fruits on a daily basis (as recommended!), depending on the hamster he will not drink much additional water. But even if you don´t see your hamster drinking, he maybe drink if you´re not looking!
The water in water-bottles should be changed every other day, or at least every third day.
Please see the Products-section, Basic for available waterbottles.
Cause hamsters are tending to dump their bedding everywhere, you should not use a bowl for water (if you don´t want to change the water three times a day...). It´s also not natural for them to drink water that way (see above). But if no bottle is available, better to use a bowl than leaving him without any water!