Well, I think this game is a lot more forgiving than the first game. (I played both game's story modes to completion) In Splatoon 2 you get armor at all times that let you take as much damage as it would normally take to kill you but still hold on. Sure movement is slower, but you can try and hide in your ink for a quick recovery (quicker than just staying in inkling form that is) (Ooh and don't forget, if your armor breaks, hitting the next checkpoint will restore it instantly so sometimes rushing past enemies is a valid tactic). I think it's kinda hard to actually die in this game. (I will admit though I did actually game over on the final boss but I was still trying to learn his patterns) I agree with that comment though, sometimes if there is a case where you do die the checkpoints can appear to be a bit spread out at times, making going back a bit frustrating. However (keep in mind that this is coming from someone who went and 1000% completed this game's story mode) I really think that the single player gets easier as you progress. You start learning typical patterns and where secrets are most likely to be. Once you learn how certain things work I found that the single player can be a blast to speedrun!
I would recommend playing the game guide free, I was able to get most of the secrets without one, (ended up using one to find 1 sunken scroll and 2 sardinium after beating the game.) but if you are getting stuck a lot, let me know, maybe I could help by giving some advice :D
(Or just look up a guide, I won't judge you ;))