Naval Tradition indeed is very mediocre, promotions/xp to naval units won't add much, especially with the weak naval AI. Why not a 25%

boost on naval units and buildings and +1

on sea resources?
I like that idea. Heck even the +1 prod to sea resources would be very useful.
Here's a thought, would it be overpowered to make the deep policy of commerce a reduction of all roads and railroads by 1 gold? In other words, roads become free and railroads cost one.
That way a person of high commerce can build roads wherever he wants (although I guess I don't know if you can actually build roads late game or if they have to be railroads)
Here are just a couple of other brainstorming ideas for commerce:
1) Luxuries provide +X gold.
2) Research agreements provide X% more research (if you are using Thal's new RA idea)
3) Gold deals provide you X% more gold.
4) Naval Ships gain +X% more attack and/or defense
5) Penalty for attacking off of embarkation is reduced by X%. Or perhaps move penalty for embarking is removed.
6) Trade Routes provide X culture for every Y amount of commerce they generate
7) Capitalism: Gain X% of your current treasury value as a bonus to gold each turn. (more gold you save, more gold you gain!)
8) Villages provide +1 gold.
9) Gain +X gold bonus from pillaging/capturing cities.
10) Blockading ships steal X gold per turn.
11) Merchant Navy: Your naval ships can go through foreign sea territory, even if you don't have open borders. (I believe the city state diplomacy mod has a promotion that can do just that)