SEA archives are Self Extracting Archives. These archives are indeed programs. On the PC, this type of archives would have likewise an "EXE" extension. You have to "run" the program to get the data decompressed.
In the case of a SEA file, the resource fork contains the "engine" which will uncompress the data (which is in the data fork, you guessed it!). The programs are always in the resource fork on the Macintosh (at least the computers with 68K processors; it's not true any more for the Power Macs, but the decompression code is still 68K-code for backward compatibility reasons).
Downloadable SEA archives are rarely stored as real Macintosh files (with both forks) but mostly as MacBinary or BinHex files.
If the archive contains only data, and if the file format exists also on the PC, see the links below. If the archive contains mainly code, the only solution to exploit it is to install the SEA archive on a Macintosh medium and to execute it.
For the time being, they are no reliable Macintosh emulators on the PC, which means that you have to run the program on a Macintosh to get the contents of the archive.
SEA files can be produced by several programs. The most frequent ones are Allume (formerly Aladdin Systems) Stuffit (which automatically adds the 'SEA' extension and CompactPro (formerly called Compactor), which doesn't add the extension.
The signatures of files created by Stuffit are SITDSIT! for the plain archive and APPLaust for the self-extracting one. Likewise, the signatures are PACTCPCT for the plain archives of CompactPro and APPLEXTR for the self-extracting ones.
There are programs on the PC which know the structure of the compressed data fork and therefore can decompress such files. Allume Systems free Stuffit Expander can be downloaded from their site. The full version of the product can even uncompress to a MacBinary container.
I also found an utility to decompress CompacPro files on the PC on Compuserve DTPForum, but was never able to locate one on the Internet.