I though about something recently...
will "naval techs" give access to some units / mecanics that can be used out of sea ?
Keep in mind that G&K is introducing a lot of new naval mechanisms that should make navies quite exciting and dangerous:
- Mellee ships that can attack and capture cities.
- Ship capture.
- IIRC, the ability to demand a ransom rather than capture a city.
- [Edit: HN unlikely because the Sea Beggar (a privateer replacement UU) doesn't have it].
Most ships in Éa are going to be melee: galleys, triremes, dromons, etc. This makes strong navies a real terror for coastal cities even without land unit backup. Of course, they won't be able to hold a coastal city adjacent to a strong civ (which is good), but they can dominate island cities or cities not well-connected to core empires. Or they can just be pirates and take the ransom.
Ranged ships are going to be rare, either very high tech (galleons with cannons) or "specialty" (arcane barge). I have no idea if it will be possible, but if G&K ships work like current land units, it will be possible for them to have
both melee and ranged attack. I might use this for something like a galleon. One assumes that they have some soldiers on-board in addition to cannons.
In a standard size Continents (or PerfectWorld3) map I think about half the cities end up being coastal. So I don't think any sort of "near the sea" abilities are needed.
Keep in mind that Bronze Working is going to give you both Triremes (an upgrade from Galleys)
and Light Infantry. For the pinnacle naval unit (Galleon) you need Navigation, Iron Working and Chemistry, which happens to also allow Medium Infantry and Cannons. So a "conquering naval civ" is going to have some pretty tough land units. (A "commerce naval civ" can forgo the metal techs, but they will probably be focusing on commerce policies and have access to mercenaries.)
Any serious "conquering civ" is going to have to go down one (or more) of the existing military tech branches. Non-conquering civs should strongly consider picking up at least one of the 2nd tier techs that gives some defensive capability (Bronze Working, Archery, Horseback Riding, Mounted Elephants or Mathematics), unless they are getting the mercenary policy (in which case they don't need to worry about military techs at all since you can hire units you don't have the tech for).