Thanks guys I'm a survival game addict and having fun so far. You are on to something here, even in it's current basic state Raft is addictive. You'll make a killing on Steam, should you decide to continue developing Raft as the quality of your game already exceeds some of what is on offer there.
Suggestions: (trying to keep the current state of development of the game in mind)
Potatoes and trees are overly easy to grow - maybe add a watering system (boiled water) to the growth cycle and seriously increase the amount of time it takes food and trees to grow. Different kinds of veg/fruit, as already mentioned, would be grand. Fruit at sea you ask? Well we already have potatoes :D
The hook and fishing rod becomes obsolete very quickly. Once you have potato plots, killed a few sharks, and have built your raft to catch all debris these items are of little use. Suggesting sharks drop less meat, (thereby keeping the fishing rod in play) and shark skin, placoid scales is a source of rawhide leather, which opens a world of opportunities for additional item crafting.
Eventually (I know this is asking a lot) - Day/night cycle as well as rain. Night time and rain decreasing visibility. The cooking station has a pretty nice base, maybe this can be modified into a permanent fireplace of some sort, utilizing wood as fuel to provide a light source on your raft during night time. Fireplace can be upgraded with a shark skin canopy to still be useful while raining. Rain can directly provide water for your plots/trees.
Fleshing out health management by introducing seaweed as a resource? (trying to keep to the lost at sea theme). Traditional Chinese medicine uses hot water extracts of seaweed in cancer and goiter treatment, not too much of a stretch I think for us to use as a source of medicine. Food/water alone should not be enough to recover from a shark bite.
The durability of tools to decrease slowly. Because scrap is easy to come by and quickly enough you start throwing it away.
Horizontal stairs, check. The amount of space needed to built the (very pretty) stairs currently is a bit much to use often. That brings me to another point. The very nice crafting mechanic in Raft begs the player to built something nice and fancy (read big). Whether horizontal or vertical. The shark mechanic of attacking in random areas forces the opposite. If you build too big, (or high) you often risk losing pieces of raft before even reaching the shark.
Two things come to mind. 1 - Shark bait. Using several fish and rope (and a coconut for buoyancy?), tying shark bait in the sea to the raft/dropping it near the raft, to attract the shark attacks to a certain area, with a limited lifetime. 2 - Some kind of repairable fortification that can withstand more than one shark attack, or requires a longer attack before being destroyed. High resource cost (lots of scrap) to limit how many you can build in a given time, thereby not entirely negating the danger of shark attacks. Other restrictions can be imposed on fortified tiles by not being able to fish over, use the hook or fill up your tin can in that area. Already I've found surrounding my raft with cheap to make nets on all sides greatly reduces the danger of sharks destroying tiles with important structures, especially pillars. Regularly rebuilding nets because I was busy on the 2nd floor and could not reach the shark in time becomes a chore after a while.