Seriously they still suck. All you need is to attack from ocean with frigates and privateers and let the turtle ship taking damage without a capacity to counter attack properly because you need tiles around to surround all units(pretty lame in mp game imo when you face 7-8 of them, you want more mobile units to counter more effectively). And in sp games it's always better to attack with frigates at this stage of the game. If you need turtle ships to defend yourself, you are doing something wrong.
If you're leaving your turtle ships where they can be attacked and not be able to attack back, THEN you are doing something wrong. If the enemy already has frigates and privateers and all you have are turtle ships then you're stupid for not at least getting privateers since it's the very next tech after turtle ships.
Also, if you have turtle ships then you are definitely playing as Korea since CS's can't gift naval or air units. Therefore, you should have enough BPT to finish Navigation in less than 10 turns after finishing Astronomy. In which case you can then use Privateers to explore. Since Privateers have 5 movement instead of just 4 like the caravel, you can explore faster with them anyway. Alternately, if you have the iron, you can build Galleasses while completing Navigation and immediately upgrade them to frigates to explore with.
It doesn't matter if you're playing MP or SP, land or naval, if you're using only melee units, or only ranged units, you have a lot more problems that dealing with an enemy keeping their naval units out of reach of your Turtle Ships.
No, it's not "always better to attack with frigates". Frigates can't capture enemy ships like the privateer can. Frigate's can't capture the city like Turtle ships and Privateers can. Actually it's always better to use the correct unit for the job. Such as using your Turtle ships to prevent any new ships from leaving the city while the ranged units soften it up. Or if there are enough tiles to do so, attacking the city with multiple coastal raider Turtles to collect some cash and make it possible to either take the city in one turn or the very next turn.
Plus if you do use a Turtle to attack the city even if you can't take it that turn, the defenses are more likely to attack the Turtle that can take more hits than the frigate behind it that can't.
In any case, you're underestimating how good the Turtle really is when used properly, because you're focusing solely on the fact that it can't enter the ocean.
Here's my vote for the day.
African Forest Elephant 21
Atlatist 21
B17 19
Ballista 20
Battering Ram 17
Berserker 18
Bowmen 20
Camel Archer 22
Carolean 23
Cataphract 17
Chu-Ko-Nu 26
Companion Cavalry 20
Conquistador 22
Cossack 18
Dromon 19
Foreign Legion 21
Hakkapeliitta 16
Hoplite 16
Horse Archer 23
Hussar 18
Hwach'a 19
Immortal 20
Jaguar 20
Janissary 22
Keshik 26
Landsknecht 19
Legion 23
Longbowman 28
Mandekalu Cavalry 16
Mehal Sefari 16
Minutemen 21
Mohawk Warrior 23
Musketeer 20
Naresuan's Elephant 21
Norwegian Ski Infantry 16
Panzer 16
Pictish Warrior 21
Quinquereme 18
Samurai 18
Sea Beggar 24
Sipahi 20
Ship of the Line 20
Slinger 9
Tercio 22
Turtle Ship 13
War Elephant 21
Turtle Ship - See the top of the post.
Battering Ram - Great for very early rushes when backed-up by horse archers. However, it's easy to kill and any promotions earned with it don't help when it's upgraded to a trebuchet.