well thats one of the ideas..you would also elimanate bugs like mites an gnats...i think it could work but you would defently need small indi plants for this an building something to house a 6 foot sativa may be a little chanlgeing...but i think there is something to it...peace
As far as i know you cannot completely submerge cannabis if you do it dies even in oxygenated water and because it is highly likely that a lot of the resin and thc is produced to keep the leaves moist and protect the plant even if you managed it the weed would be weak due to constantly being wet.
The best course of action would be to try to cross breed cannabis with an aquatic plant and hope that you could somehow pass the genetics to make thc into the naturally aquatic underwater plant but i still think that even if you achieved this the stuff would only be good for thc extraction and never for smoking like you would some normal weed.
On the upside though you could plant tons of it underwater and not get arrested and than make a shit load of thc extractions so even if the plants only produced 5% thc it would still pan out,you could grow them in rivers etc. and no one would even know lol
Dreams huh lol