Pressure differential is definitely one of the bigger problems. In a manned submersible the interior pressure is usually roughly sea level. So you've got hundre

Pressure differential is definitely one of the bigger problems. In a manned submersible the interior pressure is usually roughly sea level. So you've got hundreds of atmospheres on the outside vs one of the inside. Anything that requires some pressure differential requires a strong pressure vessel. So not just the crew compartment but also camera housing and the like. This is all compounded by the fact the deep ocean is very cold so as a submersible descends there's a thermal gradient with "survivable" on the inside and sun-zero on the outside. Temperature differentials interact with the pressures because the atmospheric pressure and low temperature will both cause material to compress/contract. The whole vessel structure needs to be able to remain structurally sound with all of that expansion and contraction. The submersible is also in salt water which corrodes different materials and can cause electrical shorts. A submersible could have a few successful missions then fail because some corrosion was missed on an inspection. Building a manned or unmanned device to travel to and return from an extreme environment is really hard. You need to understand or at least make educated guesses about the target environment, understand your building materials, your construction processes, validate and bench test as much as possible, and have the same amount of thought go into actual operations. "Fuck it we'll do it live" is not a recipe for success.