I recall reading that the United States Coast Guard and its entry into the Marine Safety realm occurred during the period when steamship boilers were routinely exploding. They started inspecting steamship boilers and found out that square boilers soldered together or welded or whatever affixment they use were not anywhere near the strength nor could they avert the pressures adequately before they blow. So as a result the advent of spherical boilers became the safest design feature tested.
The strongest submersible object would be a perfect glass sphere but as you know that is impossible. The next best is spherical with minimal seams. Titan looks like something a kid would order from the back of a 1972 Sears catalog.