I had the same issue. All my great admirals were spawned in the city with Huey Teocalli and a harbour. I have the Summer 2017 Update.
There is a solution though:
1. Open the file "Units.xml" (it is located in ..\SteamLibrary\steamapps\common\Sid Meier's Civilization VI\Base\Assets\Gameplay\Data\Units.xml)
2. Make a copy of Units.xml if you like to play it safe
3. Go to line 671 (or the line with "UnitType="UNIT_GREAT_ADMIRAL" " in the section "<Units>")
4. Change "DOMAIN_SEA" to "DOMAIN_LAND"
5. Start or restart CIV 6 and load your save game
6. Be aware that you won't see the change until the next turn
You can now move your great admiral as if it was a land unit. Including transferring it to another city.