Less active players do have the option of a 50k Free XP Submarine Container. It has a nearly 30% chance to pull a Sub, and rewards 70k Free XP if a Sub isn't dropped. That isn't going to assure a Submarine for everyone, but it is something which helps with availability.
Otherwise, a player would need to do the personal challenges to get Submarine Tokens. That's not a small task, but it's certainly doable.