I may be being dense, but I fail to see a plausible situation where you'd need to completely submerge your AEG on purpose (regularly no less!)... I mean, are you really crossing 12 foot deep rivers often at your field? I understand it would be a cool photo or whatever, the quintessential "SEAL rising out of the murky deep" comes to mind... If it was absolutely imperative that I cross water that deep, I would bring a couple heavy duty trashbags, have half your team of elite maritime waterborne operators cover you from the far bank, wrap your rifles in the trash bags, cross, and cover the others as they did likewise. Will it look as cool? No, but I can state with the utmost confidence it'll save you some headaches down the road.
I'm thinking that your biggest issues with repeated submersion is going to be short circuits and accuracy degradation. We preach cleaning your tightbore regularly to keep it from fouling, you're looking at intentionally fouling the barrel with whatever sediment and grit happens to be suspended in your river... I'm theorizing that coupled with the risk to your barrel's accuracy comes the possibility of hydrolocking. If your gritty barrel gets a bb clogged in it I have to assume that your piston will still attempt to compress the air and water in your cylinder, water being an incompressible medium and with nowhere to go something's gonna give, be it the stoppage in the barrel, your piston teeth, etc. etc.
I've seen the videos of people shooting their AEG after submerging them, but I hardly think that it's sustainable as a long term practice, both from a reliability and accuracy standpoint you are asking for trouble dunking your rifle on a regular basis.