100% seadra level 30

Save it for Kingdra...

Good, and save it for kingdra, don't power it up just for now.

Like the others said, save it for Kingdra and pray you don't end up with twister.

As for prestiging: if you have a good cloyster or dewgong: then I'd pick those over your Seadra for prestiging against Dragonite.

Edit:
I realised that Seadra can't learn twister, so Kingdra probably won't be able to learn it either.

Unfortunately the dragon moves of Seadra are so weak that its water moves do more damage even against Dragonite. Kind of stupid this is. I would like to see Hydro Pump and Watergun nerved down, that would really shake up the game...

Is it male?if it is, level it up until it envolve and learn dragon dance and prepare a female dragonite, then you can get a dratini with dragon dance, hmmm, that is all I remember, lol
Speak of kingdra, I think his advantage is no longer weak to ice compare to dragonite. So it may be a good defender rather then attacker. But, the max CP of it destroy this dream:( So, still, it is more like a pet, rather than something you can make good use of.