The Poseidon is a very slow, but heavily armored rocket cruiser that is perfect for destroying a base from the water.
Can attack almost all units with no real hard-counter
Very high range and attack power
Very slow to produce
High cost in resources and CP
Submarines
Takes very long to reach the enemy base due to its slow speed
The Poseidon is first available beginning from HQ3 and requires a big investment of both resources and time to build it in sufficient numbers, but rewards you with a supreme battle performance.
The best way to use them is when coupled together with Vikings, which cover their only weaknesses, submarines. In turn it offers a very strong anti-air missile weapon, which coupled with its huge health pool and armor, allows it to easily defend your fleet against Hawks (even in superior numbers) and can also be used as a meatshield for the less armored Vikings.
But this doesn't mean that they can't be used without their support, as the currently only submarine in the game, the Barracuda, is rarely seen, therefore often eliminating it from the equation entirely.
Furthermore even the heavy warship of the Resistance, the Alligator, while capable of beating Poseidon in a 1v1, can't do so easily, which means that a moderate numbers advantage is enough to defeat it. Poseidon excel the most when amassed in huge numbers and used in narrow passages, as this allows them to fully utilize their huge range and therefore defeat enemies that it normally wouldn't be able to beat.
For example, in a battle of 1 Poseidon vs 1 Alligator, like mentioned above, the Alligator wins, but in a narrow passage and with 10 Poseidon vs 10 Alligators there is a slight chance for them to beat the Alligators.
All this combined allows them to break through even the heaviest of defenses, especially if one considers that they also outrange all defenses except Sniper Towers, which only deal negligible damage to them.
Last but not least one should mention that it is possible to reveal the location of submarines with Cyclones and then force fire them, which allows Poseidon to damage and with sufficient numbers to even destroy them even when submerged.