You're welcome to read the manual yourself here. But for Navy personnel, I've boiled it down for you:
- You must be assigned to a US homeported ship, deploying unit, or attached to a Marine Corp force that deploys.
- You have to go away from homeport for at least 90 days at least once during the year OR you go out twice for 80 days at a time (helps out those in the SSBN community!)
If you meet that, then once you've been attached to the command for 1 year, you get a Sea Service Ribbon.
If you're just shy of the 1 year, you can waive up to 14 days.
A deployed unit is "A ship (USN, USNS) , aircraft squadron, detachment, battalion, or other unit that operates away from its assigned homeport or homebase for a period of either 90 consecutive days or two periods of at least 80 days each within a given 12-month period." So if you are assigned to a staff that deploys, or a P-3 squadron, then you can get the award.
It's documented by your Commanding Officer using a Page 13. You can download them from NAVPERS forms, located here (it's NAVPERS 1070/613). I made a sample for an ENS New Sailor that you can use as a guide, located here.
Be sure to document your ribbons. These fall through the cracks all the time. While it may seem silly, your page 13s will add up. At some point your record will get reviewed for promotion, and one of the ways that you are judged is how much sea time you have. If you've spent all those years at sea, but never document it, you look like just another land lubber to the promotion board, when you really deserve better.
Be sure too you wait till the one year mark. If you get onboard and deploy right away, you still have to spend one year attached to the command before you can put in for the ribbon. It's not normally a big deal, but if someone goes through your record with a fine-toothed comb, you don't want that person to find errors.
And one note: if you are attached to a ship homeported overseas, you don't have to meet the 90 day deployment. You automatically get a Sea Service Deployment Ribbon AND an Overseas Service Ribbon each year you are there. I found this out when I saw a LCDR that had 5 sea service and 4 overseas service ribbons, and wasn't prior enlisted. He had served as a minesweeper CO out of Bahrain for 3 years before becoming a department head.
