To many, the idea of serving in a WW2 submarine probably isn't all that appealing. I appreciate the conscription conditions were different for each nation too, but generally speaking, was being a submarine crewman something people chose to do or were such postings forced on seamen?
American submarines were crewed entirely by volunteers (and remain so today). Submarine duty was dangerous and submariners took heavier casualties than other parts of the Navy during the war, but there were still appealing reasons to do it: Higher pay, more leave when not at sea, a near-guarantee of combat, camaraderie among crews, faster paths to command (for officers), and specific technical knowledge. German U-boats were not crewed wholly by volunteers but it was still a desirable posting and many people did volunteer for it. U-boat crews were held in high esteem and treated well.
This older answer from u/kieslowskifan details some of the benefits of working on submarines. This answer from u/thefourthmaninaboat includes some more details specific to the Royal Navy.
I do not know whether British, Italian, or Japanese submariners were volunteers or not.