if you have no navy, a warring AI will often send a single submarine to pillage all your coastal districts (at least at higher difficulties). it sucks to be oon the wrong side of it. you need at least some naval melee to defend against those. once you have revealed the submarine, a bomber is pretty useful to attack it from afar. in general, if there is war near the coast, I find naval units much better than land ones: a missile cruiser or battleship can wreak havoc among land troops while being safely out of range, while submarines can destroy anything on the enemy coast without retaliation. and battleship come into play around the time ranged units become weak. playing against godlike AI, i generally have to face stronger armies, and the strategy for it is to keep them pinned while attacking them from afar. naval ranged units are the best in the modern and further eras, when the machine gun has too short a range and will manage to attack just once before getting killed.
No, I like navies more than land units.
As for airplanes, they give you free ranged attacks, so they are a good supporting force in any situation. only limitation is that carriers suck; they need to attack in melee to gain experience, while in real life staying out of the fray is the very reason for a carrier to exist. they should gain 1 experience every time their aircrafts go on mission.
