Hi!
Bashmu's theme is a sea serpent, per its worship by sailors and its secondary sea domain. It would be cool if Bashmu got the Water primary domain to go along with that.
B'aara, Agaslakku, Izdu, Kula, Gellema, the Martyrs, and most of the Dome Sages also have four primary domains, so it's not unprecedented for the Wyrm to get another primary domain.
His secondary domain is already sea so why not? Plus it makes it so that Bashmu preists can impersonate more faiths I say do this. Its a real shame you see someone use fire domain and think "Ok warrior or Bashmu"