According to Bulbapedia, Eusine comes from Celadon (I’ve never owned crystal, he mentions it being his hometown there)
I’ve just reached Celadon city today in Soulsilver, and one of the townspeople I spoke to mentions that the towns become so polluted that Grimer are appearing in the water.
Sucuine is said to purify water. That’s why Eusine wants it so badly. (He was most likely already interested in Sucuine before though, he mentions that he has been looking for the past ten years, while Celadon could have only have been polluted within the last three, but clearing Celadon’s water is why he wants to catch it. Or maybe pollutions been a steadily worsening problem in Kanto for a while.)
I dunno, I was just trying to think why someone from Kanto would be so obsessed with a johto legendary that isn’t mentioned in the Kanto games. I like this theory.