I am new to this game, I've only played it for about 14 or so hours, but I have come a pretty far way as I have a pretty large raft, and have basically mastered the entirety of it with everything I could ever need in only about two floors, and am NEVER lacking in materials, except for wood.
Here's what I've learned from building my stupidly large raft:
Note that this post was made during version 1.4, hours before the 1.5 release.
1: Items spawn 25-27 raft squares away from the initial 4 raft squares you start on
2: The item spawn width, assuming you are looking in the direction in which all items travel, is 7 raft squares to your left, and 8 raft squares to your right, counting the two raft squares in front of you.
3: Scrap is entirely useless late game unless you REALLY need a LOT of pillars, I have 2 chests full of it and I need it so little that I just toss scrap now as it has literally NO use, asides from maybe a chest if I need it.
Now for the small critiques of this awesome game:
Shark attacks are really annoying when you're on your second floor doing stuff such as cutting down your tree farm.
The drinking sound of the purified water sounds like my nose when I drink water while I have a sinus infection, and I think every time I hear it in game my nose twitches.
Scrap has little to no use what so ever late game, once you have the pillars for your second floor etc and a few chests you might as well toss it as it no longer has any value.
Other than that because the game is early in development this game is freaking awesome, you guys rock.
Also I found that there are a lot of youtubers playing this game on let's plays including doing really crazy stuff like building things stupidly high or just screwing around playing the game from nothing, which is how I found out about this game...from a youtuber's thumbnail image and thought "this game looks awesome!"