Now I'm going to play devil's advocate. I think the issue with this theme is actually that it's so abstract. Each policy is less obviously and directly related to each other than something like Rationalism.
But honestly, all this stuff is related to exploring in
one way or another:
Unlocks in the Medieval Era
Exploration because we're leaving the dark ages behind and the idea of exploring other lands is being birthed.
+1 movement and +1 sight to Naval Units.
Exploration because nations dedicating to naval explorers will focus on better means to do so
Unlocks building the Louvre
Exploration because a museum to show off foreign works of greatness will interest the population in finding out more about the rest of the world
Maritime Infrastructure; +3 Production in all Coastal Cities.
Exploration because this policy ramps up coastal production for the purposes of building exploration vessels, and hopefully one day trade vessels and harbors and ports to do the trading in
Merchant Navy; +1 gold for each Lighthouse, Harbor and Seaport; Requires Maritime Infrastructure and Naval Tradition
Exploration because exploring the seas in order to found coastal colonies is now more lucrative and easier to set up
(An interesting gameplay mechanic just jumped out at me actually that I'll post elsewhere)
Naval Tradition; +1 happiness for each Lighthouse, Harbor and Seaport.Requires Maritime Infrastructure and Merchant Navy
Same as above; coastal colonies are now more feasible
Treasure Fleets; +4 gold from all your Sea Trade Routes. Requires Merchant Navy.
Same as above; coastal colonies are now more lucrative
Navigation School; Free Great Admiral. +2 Movement for all Great Admirals. Earn Great Admirals 25% faster. Requires Naval Tradition.
Exploration because what naval exploration-based Empire can do without a fleet to support it?
Purchase Great Admirals with Faith in the Industrial Era.
Same as above
Finisher; Show hidden Antiquity Sites.
Exploration because antiquity sites refers to both indigenous and foreign sites. One would need to explore into other lands and cultures in order to find the most exotic treasures for your museums
The British chap in the Jungle
Exploration because the dude is obviously exploring.
I kinda wish this tree would let you purchase archaeologists with faith, come to think of it.
Anyway, I have to agree everything relates to the vague notion of Exploration. However, I don't have to agree that the theme is a strong one
Edit: I'll also add that I agree the link between naval exploration and "setting up monetarily lucrative coastal colonies" is totally stepping on the toes/ripping-off the Commerce tree, past and present. This weakens the theme for me as well.
Edit2: I recant that statement. That would be like saying the Liberty opener steps on the toes of Aestheticism. I think this ability makes sense actually.