I wouldn't call it hardcore, or even terribly difficult. Your underwater time is limited by your O2 gauge, which drains faster when you sprint or take damage. You do have to be more careful during early dives because you can't stay underwater for long but by midgame I found myself rarely worrying about O2. There are resources all over that give you full tank refills. Even reaching zero oxygen and getting a game over is more of an inconvenience than anything, you just resurface having lost most of your collected fish/items. It's very hard to reach a true failure state, you're always progressing somehow.

Like 18Rabbit added, some fish are aggressive but they're always marked with a red diamond. If you keep your eyes peeled, you rarely end up taking fights that you don't intend to. You also get better tools and weaponry to deal with them as you progress. The boss fights range from very easy to decently challenging; it's mostly the optional bosses that fall on the higher end of that spectrum. If you lose during a boss fight, you're able to retry from the start of the encounter.

Imo its greatest strength is that it caters to lots of different playstyles. If you want to focus on minmaxing your profits at the sushi restaurant, go wild. If you want to explore and collect, there's plenty of that too. There's challenging optional bosses and a pretty fun main storyline. Lots to enjoy without a ton of stress involved.

I’d put it about the same as Stardew. There are boss fights and there are some jerk animals that are dangerous and a PITA to kill but generally you can avoid them or develop ways to deal with them. Overall it’s a fantastic game with a great sense of humor and just when you feel like you’ve done all there is a whole new mechanic is unlocked.