>>213821261
>>213821315
If I recall the mythbusters did this and figured out that you'd need like 2,000 pounds of weight to keep the boat under.
On the topic of impractical underwater air pockets I was reminded of this movie I caught briefly once where people live in an underwater house and I think it's a comfy house. Maybe if you built it by a steep bank and had a walkable tunnel to dry land so you didn't have to swim out every time