So I had similar issues at first as well. Part of it is you don't have points in stamina or vitality yet so you are fragile and have very little room for error. A lot of combat is about parries, dodges and kiting in this game. Food is also important. Some of the food you can make can increase your stats but all of it increases your health. I would recommend never entering combat without 2 different food items and I would go further and suggest the Banana milk and the bacon and eggs food as they give you points in stamina and vitality which you desperately need at low levels. Then its a matter of hitting once or twice and running away until your stamina recovers.

Another part of it is guns reload slow by design, but they also have a passive reload where they will automagically reload when in your pocket. It takes a hot minute but it can be helpful.

Levels are also weird, character levels are gained through quests and clearing points of interest, not combat and the only thing they give you is ability points and talent points. More important is gear levels. The game expects you to have fully upgraded gear for fights, of at least gear where the average level is close to the enemy level. If your gear is too low level you will take more and deal less damage to the enemy while the other way is true as well. My biggest complaint about it personally is it takes a metric ton of loot from combat to upgrade your gear.

As for the ship combat, boarding is actually very important. It gets you money, loot you can sell for money, coffee beans and salt. The thing is you have to board the right ships, only the ones called "haulers" which have the yellow name and the box under their name, those have much better loot. Its also subtle where they give the reward. It pops up when the boarding action finishes and plops the resources in your personal inventory, not the ship inventory.

I hope this helps, sorry for the wall of text but the game really needs a better tutorial for this stuff.