This isnt an in-depth guide or whatever, but it should be useful. Ill write about other aspects of water myth than simply how well they kill ships as well, since those aspects can be very important. I wont print exactly how tests were carried out, how many hp/sec a Kraken does in damage etc etc, but I will rather draw the basic outlines of water myth creatures´ capabilities.

Starting off, lets look at heroic water myth...

Nereid, Kraken, Scylla and Leviathan.

Letting them battle each other, myth vs myth, is an important test since FHing is popular, so these units will frequently go 1v1

they rank thusly:

Nereid
Kraken
Leviathan
Scylla

Notes: Nereid wins, no surprise as it is a myth killer unit. They really gut other myth. The others were pretty close to each other.

So how about vs ships then.. arrow ships were what I tried them out vs. This is just a test of how f a s t they kill ships, the Leviathan could take on more ships than a Nereid.

rankings:

Kraken
Nereid*
Scylla
Leviathan

*nearly died in the process

Notes: Kraken killed almost only by its special, microing that special away if the attacked ship is very low on HP can be useful. Nereids make flotsam of ships, but they die much more quickly than other water myth. The Scylla didnt get enough kills to really get anything out of its headgrowing special, and the Leviathan comes out last, however it is not at all bad for a water myth unit. It just doesnt have a very powerful special.

Other considerations: Despite the Leviathan and Scylla ranking low, they are by NO means bad. For one thing, lets look at the gods you must pick: Njord is not that great, esp not for Loki, and Hyperion.. *sigh*.. OTOH, Dionysus and Nephtys are both great. Due to eclipse affecting the Leviathan, the Isis FH is even more OP on water maps.

After the free myth, Zeus of course has by far the best possibility to make more. Scyllas are cheap, and only Hyperion could beat Dionysus in a water myth war.

Next, the mythic myth...

Man o´War, Carcinos, War Turtle, Jormund Elver.

When you let them fight the Kraken 1v1, they come out as follows:

Carcinos
Man o´War
War Turtle
Jormund Elver

Notes: Surprisingly, ONLY THE CARCINOS managed to kill the Kraken! All other mythic myth died.

Going up vs ships however, it´s:

War Turtle
Jormund Elver
Carcinos
Man o´War

Notes: War turtles have their ship fling, which doesnt look impressive but is quite powerful. The Jormund Elver´s high attack pays off here, its a cheap myth unit but still made a lot of damage alone. Man o´Wars, despite using chain lightning on several ships, didnt do that well.

Other Considerations: No god here is bad. Thoth gives you even more OP elephants, Tyr is generally powerful, Hera is great and Helios also solid. You really wont want to pick any of these gods solely because of their water myth though. The Jormund elver might be the most interesting unit, since it has a HUGE attack and is ranged, so as a support unit on a map like mediterranean or even watering hole, it will rip land units apart, while siege ships hit shoreline TCs.

Other questions regarding water myth in TT:

How good is the servant?

The free servant is very nice for annoying enemy fishing ships if you dont plan on making any arrow ships in classical. Its healing capabilities are a little too slow to be really useful, but a free unit in classical is always good. You dont have to swim away fast from enemy arrow ships since the servant can take a decent amout of damage, but dont go around challenging enemy fleets with it. Servants+biremes is a more powerful combo than other classical navies, and you might be able to destroy all enemy docks and ships with it in classical, if you spend res on it.

How good is Heroic Fleet?

Just as a simple comparison, the Scylla can destroy 4 arrow ships without them having heroic fleet, but with heroic fleet it loses to 3 arrow ships. The effect is really significant, and almost obliterates the usefulness of water myth for most civs after the enemy has researched it.

Ok thats it for now, comments welcome