Long time owner, short time player - suggestions/thoughts.
So, I bought this roughly 5 years ago. I played it a tiny bit back then but it was so early in that development cycle, so unfinished, that I put it aside. I gave it another try yesterday. It has come a very long way but it’s still pretty rough around the edges and could be so much better with a few fairly easy adjustments.
I generally like the graphics. A few things need work, mostly the character models and animations, but the rest of the world looks pretty good overall. I can see down my own shirt when I’m rowing the life raft, for example, and that just feels odd. The rowing animation seems stiff, too. The sounds are decent, but I’d like more ambient noises. The sound of the ocean, wind, more animal sounds when appropriate. I like the numerous crafting options, too, but crafting could be improved with a better UI and a few more options, mostly where you should be able to substitute materials.
Biggest crafting annoyance and issue? Let me make lashings out of more than two plants at the beginning of the game. There is no reason I can’t use a palm frond instead of the tiny palm plant or Yucca plant for fiber. You should be able to easily use the fiber that is found at the base of certain palm trees, or the fiber within the husk of a coconut, to make lashings or cord. It’s the most frustrating bottleneck early on and it’s unnecessarily limiting. All I end up doing is scouring each island for those little palm plants and Yucca when I’ve got loads of other plants that I should be able to use for fiber all over the place. This drives me nuts because full size palm trees would give you so much more useful fiber, and the fiber that grows at the bottom of several species of palm trees will regrow in about a month so it’s basically an endless source given enough time.
You also should be able to use cloth, leather, and any other similar materials to make effective lashings. Pretty much anything reasonable to avoid this bottleneck… 
Why do I need to keep putting fronds or other plants in the water still? Why can’t I make a device to catch water when it rains? Why do I need lashings to put water in a coconut? I understand the lashings to make a coconut canteen with a stopper of sorts, but why can’t I just use coconuts to hold water? Dig a little spot on the sand to stand them upright? Make a plug out of wood, or other material? You have other materials that you could use to hold water. I feel like this system needs a lot more work.
Food - why am I “skinning” crab and fish before I cook it? You can, but you don’t have to.
You may have to clean them, but not necessarily skin/shell them. Why can I only cook one thing at a time on the fire unless I make a spit? There’s no reason I can’t jam all three chunks of fish or crab on the same stick, or cook them with the skin or shell on by putting them near the fire, or wrapping them in palm leaves (or other sturdy leaves) and cook them that way.
Inventory. That awful noise when I can’t pick some thing up needs to be quieter, changed. or eliminated. I’m sick of hearing it. The fact that if I’m carrying “too much” that I can’t swap items around. Little annoyances that get worse and worse as I progress through the first few days. Extremely annoying. The clunky interface. The very limited storage capacity of the crates. This all needs work. Even the inventory on the life raft is very limited, but i find it odd you can put in things, like sticks, that clearly wouldn’t fit, yet can’t put a bunch of cloth or food in there because there is no room.
Carrying capacity - I like the fact that it’s pretty low at the beginning. You’ve got pants and a shirt, a couple pockets and that’s about it so you shouldn’t be able to carry too much - but why do you need to craft a tool belt? You’re already WEARING a belt, no? Dress pants and a belt. You should be able to jam the small rock knife in a pocket, possibly the larger one, too, and the axe in your belt at the very least. A couple strands of fiber from a coconut husk and you should be able to make a couple loops or lanyards that will hold other small tools. Basically it’s harder than it should be to carry small things. Larger items? It’s fine, you shouldn’t be able to run around with too many larger items, but you should be able to make carrying bags, frames, or lashings of some sort so it’s easier to carry things. Binding a few dozen sticks together makes them much easier to carry around, for example.
One other inventory thing - I like that you can use things that are nearby for crafting, but you basically have to stand on top of them or be pointing at them to use them. Expand the radius a bit. It’s a great idea that needs work.
Movement when crouched feels odd. I think I understand what you were trying to do but it just feels strange.
Sunstroke - why can’t I make a hat out of palm fronds? Weaving fronds isn’t that hard. Making a hat would be tricky until you got the hang of it, but definitely possible. Why not make some cloth wraps to cover exposed body parts? Why not sleep in the shade during the day so you don’t get a bad sunburn? Why does the game force me to only sleep at night? I’d honestly try to rest and hydrate during the warmest part of the day in this situation and work/gather when it’s cooler. Speaking of sleeping - it seems like you can go forever with no sleep at all. Perhaps intended, but in a game that’s complex enough to poison you or make you sick if you eat the wrong kind of fish you should probably implement some kind of rest/sleep mechanic.
Attacking enemies - why is it so hard to hit things with the axe or other weapon? Why do I have to be right in their face, an inch away, to hit them? Most of the attack ranges feel wrong, I can hit you from 9 to 10 feet away with a long spear, and at least a few feet away with a short spear but in this game you have to be about a foot away from enemies to hit them.
Why can’t I salvage more resources from the shipwrecks? A bit of metal to make a spear tip or to make some metal arrowheads would go a long way. Why can I grab a huge wooden crate but I can’t grab a metal locker from the wall? That locker has a lot of usable metal that would come in handy in a survival situation, as would just about any metal from the dozens of shipwrecks. I understand why you can’t salvage most of it, no tools, but there should be some parts you can grab. And wiring, there would be so much wiring on some of those ships that you could use as cord or lashings, but no. You can’t really grab anything unless it’s an item in a container.
I like the game, it’s a bit clunky and still needs a lot of work but it’s certainly a decent survival game experience so far. I hope you sort out the multiplayer issues because this could be great fun to play co-op. I also hope you sort out some of the bugs, issues, and shortcomings the game still has.