Surface gravity on Europa isn't very high. So having the thing being extremely heavy would not be much of an issue if it's not supposed to take off and land on Earth. Once the sub is in the water it will be bouyant, that helps with dealing with weight too. But even with low gravity, the extreme depth of the ocean will lead to incredible pressures near the bottom, where the cool stuff is likely to be found. So the sub will have to be extremely strongly build.

Hard sci-fi, you say? Excellent features could be battling cancer thanks to cosmic rays, and the disintegration of skeletal mass and deteriorating vision thanks to zero gravity. Punishing and crushingly repetitive schedules of medical therapy and physical exercise, plus batteries of psychological tests could add to the excitement of an expedition!

Or are the crew going to be far more fit for space travel than early humans?

Will their bodies be entirely synthetic, and their minds simply transmitted to the exploration craft after it arrives? Or will they be genetically engineered, proof against the rigours of interplanetary exploration? Will they be human at all?

Or could they be disposable human workers employed by an unaccountable interplanetary corporation, who are holding their nearest and dearest hostage to ensure their compliance?

Short version: if you want to go genuinely hard sci-fi, things may look very, very different to how they're depicted in TV space operas.

For a thread of the conversation on the same topic I had in Mastodon you could follow this link, surely you will find the activity and the links very useful.