These are great ideas. Adverse weather effects and a day/night cycle are fantastic additions. Needing to seek shade during harsh sunlight and warmth during cold nights is realistic - with rain/snow exacerbating the issue.
Scripted-events would be a great addition. As you progress, certain resources should begin to dwindle. These events would provide a way of obtaining those resources but would only appear after a set amount of play time. Here are a few examples:
1. You drift toward a derelict raft. You need to determine whether or not its worth the risk of leaving your raft to explore this new raft. Maybe there are enemies hidden on-board or sharks surrounding it. There's also the risk of your raft drifting too far away to return.
2. You land at a small island. You can explore it for resources but you risk encountering dangerous animals. The island doesn't generate important resources. It MUST be harder to survive on an island than floating on the raft.
3. You float over an undersea cavern which can be explored after dropping an anchor. There could be treasure inside but also enemies and possible drowning concerns. Maybe even traps.
4. You approach a whirlpool which will destroy your raft if you don't steer around it.
5. You're approached by a militant raft with human enemies that use fire to damage your raft.
6. You're approached by a friendly raft that has armed or unarmed traders or NPCs.
I'm not totally on-board with armor as it would likely tunnel the game progression toward reaching that safety net.