If a cleric takes the aquatic (druid) domain, granting him the Sealord power (channel energy to heal or command aquatic creatures), is that in addition to his normal cleric's channel energy uses?
I think it's an option for the existing uses, not an extra pool (pun intended) of channels.
Yes.
I agree. These channels are in addition to your regular channels. This is no different than gaining Channel Energy from a different class source (for example, a Paladin/Cleric, or Cleric/Oracle).