Do take a look at my guide in this section. It'll explian everything you need about breeding, from basics like the egg's species always being the same as the female parrnts (unless Ditto is in town), to advanced stuff such as breeding TM moves (as well as a link to an episode you need for that). It also lists breeding chains you'll need.
The 3 IMO best Water types in the game are Gorebyss, Feraligatr and Huntail. I'd recommend Feraligatr since it can take it's time setting up, should it need to outstall the last sun turn or 2.
Feraligatr: Ice Punch, Dragon Dance, Waterfall, Crunch. I realky don't think I need to point out that it's ability should be Sheer Force. Also, use the search feature, there's a Life Orb in the game, which gives no recoil of you've got Sheer Force.
Gorebyss: Surf/Hydro Pump, Ice Beam (pre-E13), Psychic/HP Grass/Electric, Shell Smash. Hidden Power is much better, since only Water types resist both water and ice, and they're the only ones you'll ever use your third attack for.
Huntail: inferior to the above 2, but still great. Waterfall, Ice Fang, Sucker Punch, Shell Smash.
For an Aggron swap, Metagross isn't too great for ya since you're very Ground weak. You could use it (Zen Headbutt, Meteor Mash, Agility, Brick Break/Hammer Arm), but I'd definitely recommend either Archeops or Durant. Archeops is much faster and has godly coverage (EQ(pre-E13), Rock Slide, Acrobatics, Dragon Claw), Durant has essentially 190 base Attack due to Hustle, is way faster and has a boosting move (Hone Claws, Rock Slide, Iron Head, X-Scissor). My Durant swept even freaking Charlotte, need I say more?