.sea files are Self-Extracting-Archives, and are basically Mac executable programs. You will need to open it on a Mac to decompress it (similar to zip files on a PC).
One problem I've come across in the past is if you have the file on a PC and then bring it to a Mac, it doesn't work anymore (Mac files use a Resource Fork to determine file types, not extensions, and this is lost if it's taken off the Mac). To prevent this happening on most modern Mac OSs, rename it on the PC to have the file extension '.bin' (without the quotes). This has to be done before it gets near the Mac, and then when you double-click it on the Mac, it should work.
