That was the original idea, similar to the zep mission: one defends, one tries to bring it down.
But the issue is that, unlike the zep, the submarine is restricted to the lake and it provides a huge advantage to one of the sides. If germans got to defend it, they are near it compared to the allied side. If i switch places, the allied would need to defend it but they are way too far.
Then I tried flipping with lakes, same issue because one lake is way far from the other base and so on. At the end the best combination was to have 2 submarines in the same lake and let them submerge more often so that people would have enough time to fly to the area no matter which team they were assigned and do a little dogfighting while they waited to reappear.
As for the machine gun, things sound "cool" but the reality is that almost nobody uses them. Adding guns to the carriers for instance was requested a million times, I added them. How many times do you see people using those guns? Rarely. They do, but about 3% of the carrier games. People prefer to fly, not to land and use machine guns on them. And that's in a carrier that is designed for people to take off and land on it. A submarine is not a place to land your airplane, to begin with.
I'm also giving thought to the possibility of the submarine launching a missile attack, but it's still in progress.