How do you make health potions?
For that, you need a fire, fresh water, empty coconuts, and purple flowers.
Go to the fire in your shelter, click 'cook', go to the second page and choose 'boil the water', and then you'll have a coconut bowl with cooked water.
Then click the fire and click 'cook' again, choose 'make health potion', and a health potion should appear in your inventory.
But, keep in mind that the game is a little bugged here. You'll get no message or anything when you receive a cooked water bowl or a health potion, so it seems like nothing is happening. But if you check your inventory, you'll see that the items are there.
One more question, is the cave at the top of the waterfall bugged? I cleared the rock in front of it but cant go in. I seem to be stuck, I made the chicken coop, grew a garden, had a harvest of crops, a date with Sunday and handjobsbut I can't seem to explore anywhere else.
The cave isn't bugged, but it's not accessible until you start doing the propeller quest.
And the propeller quest won't trigger until you've finished the cowgirl quest.
But the cowgirl quest is easy to trigger: go to the beach left of the place with your shelter, and you'll see a cardboard box floating in the water. Take it, and open it (just click 'use item' in the inventory menu). You'll get a bunch of objects, including a bag of cereal. Try to eat the cereal, and then the protagonist will remark that she doesn't have any milk to go with it. Then she'll remark that she should ask Sunday about where she can find any milk. Talk to Sunday, and that will trigger the cowgirl quest.
The cowgirl can be found in a field on the island where you found Sunday. But this field is also not accessible before you trigger the cowgirl quest, so it's easy to overlook.
Once you come back with the cowgirl in tow, an event should trigger, and it should be easy enough to get from there to the start of the propeller quest.