Hey Mike
THanks for the post. First, no you don't need imagemagick unless you're creating the tiles.
In response to the tiles that fail to download: does it actually download some tiles? I'm wondering if it's downloading all of the tiles over the area that you are viewing, then failing to download tiles for areas that simply don't have NOAA nautical chart coverage. To save space on my web server, I'm not uploading blank images for tiles that don't exist. In fact, I should be doing something more sophisticated where if there is no tile at a location, it will give you back a default "no data for this location" image.
Usually when it claims that it failed to download x number of files, that's okay. It's simply saying that there's no coverage certain areas. So... are you seeing tiles over United States coastline? If so, it's probably working fine.
Thanks,
Rob