Looking at the triggers to gain the Sailor trait, any battle won by a fleet commanded by an Admiral should add 1 threshold point, with 1 point giving Naval Command 1, 2 points giving Naval Command 2, 3 points giving Naval Command 3, etc.

If the points aren't being awarded, I thought of a few possibilities:
1) Is the fleet carrying a leader or another type of agent? There might be a bug where the passengers get processed before the admiral for possible event triggers, effectively masking him, and since only admirals can be given points towards the Sailor trait no points get awarded.
2) The Sailor trait has a Characters value of family, perhaps this should be admiral, the same as other agent-only traits are assassin, diplomat and spy as appropriate.

Both are easy to test, go fight a naval battle with no passengers for case 1, and edit the export_descr_character_traits.txt file for case 2. In the latter case, Landlubber would also need to be amended.