Originally posted by Trike:

Have yet to discover what kills you in the ocean on a raft, but okay.

Well, not to spoil anything, but a general warning: the higher level biomes have stuff that can hurt the boat and may venture out from their shore a ways. Stay off unfamiliar shores and check it out slowly and carefully. A bow is your friend, you can shoot from the boat in motion and drive off the bad guys. You do NOT want to die in a boat in the middle of the ocean. Its a big hassle.

The swamp trees have a sickly, twisted and 'dead' shape to them. Once you see it you can spot swamp from the ocean from quite some distance.
Large tall rocks and light brown dirt is the plains, very dangerous.
Mountains are obvious, and rarely touch shore anyway.
Mistlands will fog up the ocean before you get to shore, turn around and go another way. Youll know it when you see it; the fog isn't like the normal stuff and you will not see 3 meters in it.

I HIGHLY suggest that when you find a swamp, scoot along the shore and find a meadow or black forest near it and set up a little camp there. I like to raise a small ring of dirt, drop a campfire and a rug and a portal, chest there. Tear up your boat and put it in the chest. Set the portal up so you can go to and from your home. Then go check out the swamp. Boating right up to the swamp is a good way to learn about why we all hate on the raft.