In general I don't invest in Exploration often because:
1) Aesthetics does the tourism and culture thing way better imo.
2) I won't always be able to settle coastal cities, and it takes 3 policies from the Exploration tree to fully maximize on them (though admittedly the 3 policies are very nice once I'm in that situation). I'd rather not be obligated to settle coastal cities either, and I don't think there are other social policies that are conditional like this.
3) +1 move and +1 vision for naval units is a cool opener, but not cool enough to make up for the rest of the tree. If I wanted those bonuses so badly, I would've bee-lined Great Lighthouse. It's more of a warmonger bonus as well, but the rest of the tree doesn't really deal with warmongering.
4) Great Admirals, the Lourve and Exploration's finisher are not all that exciting. I don't know if I even want the artifacts that badly, I think I'd rather work on getting Great Artists more often tbh.
One change I would make is to combine Maritime Infrastructure, Naval Tradition and part of Merchant Navy into one policy that gives "+1
happiness, +1
production and +1
gold for each Harbor, Seaport or Lighthouse". The resulting policy is fairly strong, but is dependent on having a coastal city in the first place, which is usually less productive than a land bound city. Making the +3
hammers from Maritime Infrastructure conditional on naval buildings also makes this a little weaker than just "Maritime Infrastructure with sometimes additional gold and happiness". This change also opens up 1.5 slots for different policies if we're still working with the 5 policies per tree limit so it wouldn't just end here.
Second change I would probably make would be to make the opener be "+1 Sight for all units", or else if we don't mind making Exploration overpowered, "+1 Sight for all units and +1 Move for all Naval units and Explorers". I would like to see the latter but the former seems a little more sane. Either change would at least make opening Exploration useful for more than just naval units (it would also make it easier to find hidden antiquity sites, but I assure you that's just a nice side effect).
I really have no idea what the heck to do with Great Admirals. Could always just add more movement or a higher combat bonus but that still doesn't address the low utility of GAs =\
I like AnonymousSpeed's suggestion of making artifacts from hidden antiquity sites do more than what a normal artifact would do. A small boost to science or culture when the artifact is picked up would be interesting and be a good incentive to go and get them up... before someone else does!