I feel you should get a % for being near a civ that has the discovery and a % for having a trade route to that civ. Natirally a % for doing the eureka/inspiration task but some are so gamey.... kill a unit with a musket man to help discover frigates?.... as opposed to having discovered field cannons which you just bang in a ship and with a bit of carpentry and shipwright skill you have a frigate.
It is hard to hide a civic from visitors
Actually, the "required additional Tech" would be represented by Bombards - heavy cast iron smooth bore cannon which formed the basis (with the short-carriage invented by the English in the late 16th century) for all ship-board cannon until the early 19th century.
IF the purpose of the Eureka/Bonus system was to 'guide' your research in given directions, it has failed spectacularly: individual Eurekas are too easy to get, research times are too short and the number of Techs or Civics being 'researched' is too low for the length of game. So we get Tech Trees that are finished to Future Tech before the game is more than half over (Turn 250 out of 500 on Standard Speed) and you research whatever's fastest, knowing that you will probably stumble across the bonuses for something else without actually having to concentrate on them at all. Unless you are aiming for a specific Wonder or UU, there's little reason to concentrate on anything.
My suggestion, made in another Thread Long Ago and Not-So-Far Away, is to First, have Eurekas and Bonuses actually related to what you are researching. Like, Kill A Unit With A Bombard for Square Rigging to give you Frigates, because without Bombard technology, your Frigates will be armed with nothing heavier than 12 pounders or catapults (historically, they had 18 to 24 pounders, the same guns used on land as Siege Weapons in the 17th - 18th centuries). Second, instead of a single 'Bonus' giving 40% or whatever percentage, have 3 Eurekas for each Tech, and whatever order you get them, they give 10%, then 15%, then 20%. So, to get a full 45% Bonus, you have to really concentrate on that Tech's bonus activities.
For examples:
Horseback Riding
Eurekas:
Build Pastures on Two Horse resources
Have 3 Chariot Units
Have a Unit attacked by a Mounted Non-Chariot Unit
In other words, have the required Resource, have a bunch of units that need to be replaced, and have a nasty example of what you need.
Currency
Eurekas:
Have a Mine on a Silver or Gold resource
Have a Trade Route to a foreign city (Civ or City State)
Trade a Resource to/from another Civ.
Engineering
Eurekas:
Build at least one
Ancient Walls
Attack a Walled City (Civ or City State)
Have a City with a population of 10 or more.
I hope it's clear: you have to have a demonstrated Need for the Tech and the Resources to exploit the Tech, to cause your population to concentrate their efforts and get the % bonus for the Tech.
I think that would solve the concentration problem with Eurekas and Techs, but the Speed Problem could only be solved by increasing the cost of the Techs (and Civics) so that it actually takes some decent percentage of the total allotted game time to finish the Tech and Civics Trees.