I don't..... understand what you're asking. Possible language barrier aside, I'm not sure you know all the pieces you're talking about. A Builder is a single unit. When embarked, it changes its model into a boat, and can build improvements. A Trader is also a single unit. It also has 2 forms, whether it's used for a water or land trade route. Both of these units can have boat models, but they are very different, distinct units. A 'Boat' is simply not something that exists in the lexicon of Civ 6.
From what I can guess, you want to restrict a civ's trade routes, or maybe a custom Trader unit, to only being able to establish naval trade routes, and be completely unable to establish land routes. If you were to edit the Trade Route UI menu, you could absolutely restrict a player from selecting a land trade route, but this would likely cause a number of problems: firstly, this makes your mod much less compatible with other mods, as two mods can't change the same base game lua file; second, this will NOT restrict the AI from selecting land trade routes! Unfortunately, I know of no way to restrict trade routes in this way, and there is no effect in the base game even to affect the range of trade routes. If it's a thing you absolutely need, and don't mind the extra problems, you may well look into changing the UI, as you suggested. There's at least a couple mods that edit that menu, you can use those as a starting point.
