According to the FamilySearch Wiki (found here), the 1930 census was the first to include seamen on merchant vessels. Census day was April 1st that year. The information contained on the census sheet includes the following:
- The name of the vessel.
- The owner and address.
- What port.
- The name of the seaman.
- The state or country of birth.
- Are they a naturalized citizen or an alien.
- His occupation.
- If he is a veteran, and if so, what war.
- Address of next of kin!!
There is also a searchable page indexed at Ancestry, (found here) and corresponds with NARA publication: M1932.
You may also find an index on FamilySearch. (found here)
The following states were included in this record set.
Here are some images to help you see the results better. Most of the records I looked at were typewritten, and some were handwritten.
So, if you have an ancestor who is missing on the regular census records for his family, it may be beneficial to see if he was actually a sailor or seaman.