Seadra advice on prestiging

if nothing else, save it for a kingdra

If you go inside your local cities, you may see a lot of Dragonites in the gyms.

Seadra DB/DP isn't especially good against Dragonite. Dragonite is sensitive to dragon moves and they do 25% more damage, but Seadra isn't a dragon type so his dragon moves don't get the 25% STAB which neutralizes it. Ice types with ice moves like Cloyster, Lapras or Dewgong are much better, they get 25% STAB and Dragonite is double sensitive so they do 25%^3 = 95% more damage.