It all depends on your play style. I normally wear the battle armor of posidoen which gives you as much life pts as possible. I also equip the relic of regenration with the blade of posideon. This gives you a healthy balance of magical power, physical damage and physical resistance.
Posideon is my favorite class!!!!!!!!!!!
Personally I think if you are playing Poseidon and are wearing armour, you are doing it wrong.
As a loyal servant of your younger yet more important brother Zeus, the most effective way to serve the God of the Sea is to show your reverence towards us. By abandoning your protective gear and putting aside such trivial habits such as "attacks" and "moving", you make yourselves easier for servants of the greatest Olympian to appease the most valued deity.
After all, your role is to support your fellow combatants and in what better way can you achieve that goal than by sharing your valuable XP and less valuable lives with us?