How about creek water from outside the ryokon. Judging from the overview, it seems to run quite close, so it may be possible to sneak out without anyone noticing. Since it is the creek water that feeds into Hell's Pond, it could work... but now I'll need to figure out if any of the suspects had the opportunity to do so. Afro man obviously did, but what about the other too...Abs. wrote: Introduction of tap water into the solution would affect the composition such that it wouldn't match the pond water.
Oh wait! The rain, it rained! Any of the suspects could leave a bucket outside and collect almost distilled water that way. That would work and wouldn't require going outside!
Water Transportation: Tokubi Rokurou can make a little under a liter using the supplies for his watercolors. There are 16 of those 15 mL bottles. 16*15 mLs = 240mLs. There are 9 of the 60 mL large bottles. 9*60 mLs = 560 mLs + 240 mLs = 800 mLs. The 100 mL squirt bottle can hold water without emitting a stench. (Squirt bottles are often used in laboratories to hold ethanol or acetone which you can smell easily if they are free to vaporize - when stored in these bottles you can't smell them.) 100 mLs + 800 mLs = 900 mLs. The white container used to hold paint: remember the shoe had green stains on it, and the shoe wouldn't fit in the jar, so the paint solids must have been stored next to or perhaps inside the shoe while that container was being used to ferry water. I am going to guess the jar has about 75mLs of room. 75mLs + 900 mLs = 975 mLs total ≈ 1 liter. 
Water Transportation: Araiwa Kazuki has two 500 mL plastic drink bottles, one of which is still sealed, and a pillow that can be inflated with water. It would be hard to doctor a seal on a bottle, so it is likely that only one bottle was available for him to use. It is unlikely he prepared two sets of bottles. Araiwa probably removed everything from his bag at the creek based on the view inside his empty bag after he fell in the creek. He didn't go out and purchase identical bottles after falling in the creek and if the ryokon sold identical bottled drinks, we would have been given this information. The inflatable pillow could have held more than enough stinky pond water, but as Ran noted it was flat at the river. Also, if Araiwa used the pillow, I would imagine he would be less forward and cheerful about telling the police all about it. If Araiwa were transporting Hell's Pond water, he could only bring at most 500 mLs before the time they arrived at the ryokon. If Araiwa wanted to get more, he would have needed to make a second trip after dark. Of the suspects, only Nohira was noted to have left after Kogoro arrived. Araiwa, who could not have had a liter of Hell's Pond water on him when he arrived at the Ryokon, could not have brought any more, so unless he diluted 500 mL, he is ruled out as a suspect.
Lack of Alibi: Nohira Bousuke was on the phone with his editor going "back and forth."