Hi Jim,
First suggestion would be to buy this.
https://www.amazon.com/New-Get-Rid-B...all+boat+odors
I doubt that you are getting the smell from the bowl. Most likely it is the raw water that is in the intake line between the seacock and the bowl. As Xsbanks mentioned, using fresh water as a final flush can help. However, you need to get the fresh water into the line close to the seacock to eliminate most of the seawater from the intake line.
Peggie describes how to do it in her book above. Buy the book, it will be well worth it.