I don't think it'd flood, I think motion would bring some liquid in though if a physical object of enough mass pushed through. Or the gate would act as if it was buried and there wouldn't be a connection.
I think in one of the Russian Stargate episodes you see that the gate . IIRC they send a malp through the gate to the other side, been awhile since I’ve sent it so I could be wrong.
I thought that Barry Gates still work. They just only create a small bubble that you can't get out of.
But anyways what I mean is is wouldn't the water flow through it if you dialed out and if someone dialed in and then you submerge the gate when the water just be eaten by the gate?
I'm pretty sure properly buried gates with no void space just won't lock. That's a pretty interesting question though I think your guess is as good as mine.