When I got the x-pac I was curious to how the water units did. Considering Atlanteans can build a water unit in every age excluding archaic. As I recall my ranking of myth unit vs. myth unit was something like this
1. Jormund
2. Carcinos
Nereid
3. Kraken
4. Scylla
5. Man O War
6. War Turtle
7. Leviathan
8. Servant
Reasons:
One on one Jormund can take out any of the water myth units, including Nereid. Why, because Jormund has a huge range with a massive attack. Plus Jormunds are not extremely expensive, when compared.
Carcinos took out all below it and tied with the Nereid, due to its boiling blood when it dies. Nereid technically killed the Carcinos, but Carcinos took Nereid with him to hell. Go Hera!
Kraken is relatively good against ships, but has a painfully slow attack. Carcinos could get in 3 hits to Kraken's one!
Scylla is great, but you hardly get those 5 heads so as to multiply the damage. The only way to heal Scylla is through Athena's Restoration, so this took a huge toll on the rating. A good balance would be to change the Bacchanalia god tech from small HP gain, to allowing MUs vampirism just like trolls. It would partially offset the Greek's main weakness in the healing category.
Man O War is ok, he just can't stand his own ground. To much protection is needed for Man O War. A boost in crush armor would help it against those fast, ramming ships. Seriously, you would think the blunts hits would bounce of the jelly.
The Egyptians creatures just suck in the water and that is all there is to it. Levithan has a nice feature, but I can build ships at a much cheaper price. War Turtle will destroy ships, but outside Servant which ones really don't.
Servant is just a weak MU. He has good speed, but his heal rate is too slow. I think an increase in his heal rate and crush attack would be more beneficial.
When ranking all the units across the board, my favorite lies with Carcinos. While not the prettiest or the flashiest, it gets the job done. Carcinos has good armor with a balanced attack, great speed, and the boiling blood is a great feature. While Jormund definitely is great, if you can get inside the Jormund, it will fall with ease. Carcinos is just a pain to take out, with great attack speed that usually can get at least two hits in to your one. And not to forget, you run the risk of killing yourself in the process of killing the feared Crab.