I got Subnautica for free when it was given out by Epic Games during the ongoing free-game-a-fortnight. It was so much better than I was expecting, extremely immersive and exciting, and actually has a surprisingly good storyline that you kind of participate in without really even noticing you're progressing the story.
I also really liked Terraria, which I paid the full $10 for, but it frequently goes on sale for $5 (personally I think it's worth the full $10). It's similar to Subnautica in the building and exploration sense, but the graphics and gameplay are extremely different. In fact in Subnautica there isn't really any combat (unlike Terraria), you're mostly just exploring and crafting things to help you explore.
I think both have been compared to Minecraft, but I haven't played Minecraft so I wouldn't know if the comparisons are accurate; based on what I've heard about Minecraft I'd say there are a lot of similar elements, but of course there would be unique things in both Subnautica and Terraria.
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