Complex sentence: With the bow planes at a maximum down angle, the crew would then flood the forward ballast tanks of the submarine and, often, have all available crewmembers move as far forward in the boat as practical (a "trim party").
Split into simpler sentences: With the bow planes at a maximum down angle, the crew then flooded the forward ballast tanks. Often, all available crew members moved as far forward in the boat as practical (a "trim party").