Question

How do I get horsea and it's evolutions in Pokemon mooon?

Answer 1

Using using Island Scan on a Wednesday on Melemele Island.
http://www.serebii.net/sunmoon/dailyevent.shtml

Answer 2

You cannot. You must wait for the pokebank update and transfer.

Sources: http://pokemondb.net/pokedex/horsea
http://pokemondb.net/pokedex/seadra
http://pokemondb.net/pokedex/kingdra