I want to join Marine Recon but Im a poor water treader. How can I get to where I can tread for 40+ minutes?

Practice. Just keep egg beating in the water, get used to the water, and build up your leg strength. I used to play water polo and it just took a lot of practice and motivation. Don't expect to tread that long easily within a few days, it takes a lot of time to get water oriented. A good workout for your legs are wall sits or lunges, those really helped me.