I was able to change the Portuguese marine model that appears on the ships - but I haven't been able to make them recruitable in the game itself. I will show you how I did it.
You need to create a mod with the following tables:"unit stats naval crew tables", "warscape_animated_lod_tables" and "warscape_animated_tables":
Then you need to open DB Editor and use it to create a table each of the following: "unit_stats_naval_crew_tables", "warscape animated lod tables" and "warscape animated tables". Open the "units_stats_naval_crew_tables" and select the row that contains the marines for the US. Copy it, and then go to "File< Create new unit_stats_naval_crew_tables". You will be asked to either enter the tables into a new mod file or an existing one. Choose new and give it a name.
For example, here is the row controlling the textures of the Portuguese marines on ships in Naval Battles. In the "Model" field, enter the name of your custom texture. Mine is called "TercodoArmada" and I made it in Photoshop. Then go to File <Save. That saves that table to you new mod.Next, open the "Warscape animated tables" in DB Editor and click "File< Create new warscape_animated_tables" and choose to insert it into the same new mod as the last new table. There will be three columns. In column one, type the name of your texture file (without the diffuse/normal/gloss endings), and in column 2, type the path to the file (i.e. UnitModels/Textures/nameoffile. Then click File<Save. One more table to go. Next, open the "Warscape Animated Tables" and create a new table and again choose to put it into your new mod file. There should be four LOD (level of detail) model files - either custom mesh files that you imported in your mod's "UnitModels" folder or an existing default model in the game. You need four lines. In the "Path" column, type the path to your model (not the texture but the LOD files - unlike last time) as shown below. In the "Unknown 3" column, type the filename (without the diffuse/normal/gloss file extensions) of the texture you want to apply to the model.The pictures you have seen are how I did it to change the Portuguese marines to those below, which are more historically accurate than those in the game: