Well, after trying everything, I finally got the shell casing. What I thought was a boat apparently wasn't, so I have been focusing on the wrong thing. I tried the other thing that was hanging up and it fell down and over like the walkthrough said it would, so apparently THAT was the skiff. I never actually saw the shell but I kept moving the stylus around and eventually was able to pick it up.
This is a good DS game, just like a regular PC adventure game, but for me, it is hard to see things.
Thanks everyone for the help. I know that I will be stuck again so be prepared to hear from me again. I may take a break from it if Art of Murder is in my mailbox when I get home. Can't wait to start that!