The submersibles need to be less dense than water on average so they rise when you release ballast. It's always a cost tradeoff - I think Challenger Deep (Came

The submersibles need to be less dense than water on average so they rise when you release ballast. It's always a cost tradeoff - I think Challenger Deep (Cameron's former sub) was steel construction which required a large volume of syntactic foam that can resist high pressures to counteract the weight of the steel. That makes for a much larger vessel. Titan's submersible for exploring the Mariana Trench I believe is largely titanium so much lighter than steel, but at a hugely increased cost.