[ZeoRangerV]

I just assumed that

a) The mass flooding would give Serrator some edge in terms of power and he'd be able to take Xandred. Granted the show hasn't really conveyed that, but I've learnt that Samurai isn't the best at conveying things in general.

or

b) He's just another overconfident supervillain and hasn't really thought it through that far. Xandred's out of the picture for now and that's good enough for him. Given Serrator's come across as slightly cocky all the way through the show, this is what I'm thinking.