This is my calculation:
A standard size whipped cream charger cartridge contains 7.4 grams of nitrous oxide. Ref:
here.
Now 0.004% of 7.4 grams is 0.3 mg. So the dose absorbed when you breathe N2O into your lungs is just 0.3 mg of N2O.
Let's see how much nitrous oxide water from the soda syphon we would need to get the same 0.3 mg dose:
N20 is pretty soluble in water. At normal atmospheric pressure and temperature, 1.5 grams of N2O will dissolve into 1 liter of water. Ref:
here. At the 10 atmosphere pressure inside a soda syphon, much more will dissolve.
So if we assume that most of the 7.4 grams of nitrous oxide will dissolve into the 1 liter of water in a soda syphon, then we are going to get about 7.4 mg of N2O in each 1 ml of water in the soda syphon. That corresponds to about 0.4 ml of N2O per drop of water from the soda syphon (there are 20 drops per ml).
Since we established that the dose of N2O absorbed in the lungs is 0.3 mg, we will therefore get roughly the same dose from 1 drop of nitrous oxide water, from a soda syphon which was charged with an N2O cartridge.
In practice, the nitrous oxide in the water may not pass efficiently from the stomach to the bloodstream, so it may be that you need to swallow more than 1 drop of this water to get your N2O dose.
Alternatively, you could place the drop of nitrous oxide water under your tongue, where the N2O may be absorbed.