I'm not sure what a "Royal Marine" was but Find My Past seem to have some naval records. Whether that includes a signature or not, you would need to check for yourself.
There are people who visit Kew who may be able to assist, or Kew themselves may offer a research service, and if you are sure they hold the records, and you only need a quick lookup, the cost may not be too bad. It would not hurt to sent them an enquiry.
What year did he marry and what was his occupation on the certificate?
I'm sure someone will be able to help you further.