Great admirals need a movement rate that scales with age. 4 movement in the industrial and modern age is far too slow
That just makes it a Citadel that extends into the water. (Which is fine if the nearby city is set back in a narrow bay that has an ocean channel to it.) To be nautical feature, it should placed such that there is only one water tile between it and the coast. That would make it a fort similar to Fort Sumter, and it would be doing what a coastal battery is supposed to do: establish a ZOC that regulates naval traffic and defends against maritime attackers.
Two easy-to-implement tweaks that i believe can make great admiral relevant, if situationally so:
in the exploration tree, swap navigation school and the 1happy per lighthouse/harbor seaport policies around. What made the free great admiral very useful in G&K's commerce tree was that you could get him well before reseaching astronomy and send him exploring the oceans in relative safety as long as he ends his turn in an ocean tile. where NS is in the exploration tree it's very unlikely that you wont have astronomy before getting him. i also think 1happy per Lh/harbour/seaport is strong enough for a coastally focused civ to justify the relative lateness.
Secondly, what i think would be a very useful passive ability for a great admiral is to enable adjacent naval units to heal 15hp per turn outside friendly territory (not automatically, but like a temporary supply promo). i find that early naval scouting is often limited by the rate of attrition your triremes suffer due to barbs and having no access t friendly territory - i feel that this adjustment would make a great admiral not, essential, but at least somewhat more purposeful.
Great Admirals can not be underpowered compared to other GP's. They come from a completely different pool, and don't need to be balanced to same degree of usefullness as for instance a Great Scientist.
The argument of balancing and underpowered Great Person in this thread was earlier used to boost the Great Generals, and it has lead us to citadels used offensively and as Culture Bombs. Really hate that design. Worst thing about this otherwise excellent game.
So no! There is no need to boost the Great Admirals, at least not because of gameplay balancing.
Don't GAs allow ships near them to heal? I'd say that's a pretty great ability when you're a long, long way away from a friendly port and under fire.
Still some kind of tile improvement like the other GPersons have would be nice.
I quite agree with this. However, then they have to create some action that expends the GA, just like ALL of the other GPs get used up by exercising a special ability. But if they did both those things, the GA goes from being under-powered to being over-powered, because it then has TWO primary functions (leadership AND unit healing) and one secondary function.True, but your GA is burned by healing up surrounding ships. If your ships could always heal a bit in the presence of a GA without burning him up, that should do it.
I quite agree with this. However, then they have to create some action that expends the GA, just like ALL of the other GPs get used up by exercising a special ability. But if they did both those things, the GA goes from being under-powered to being over-powered, because it then has TWO primary functions (leadership AND unit healing) and one secondary function.