You get the raft in Dungeon Level 3. To get to Dungeon Level 3, from your starting point:

Up 1 screen
Left 4 screens
Down 1 screen
Right 1 screen.

Once inside Dungeon Level 3, to get to the raft from your starting point:

Left 1 screen
Up 2 screens
Left 2 screens
Down 1 screen.

Here is a gif map of the dungeon, if it helps clarify:

http://db.gamefaqs.com/console/nes/..._dungeon_03.gif

Good luck!

(Link courtesy of www.gamefaqs.com)

3 DUNGEAN