Swampert's also my Favorite (Starter/Water) Pokemon! (First Pokemon I chose when I first played Pokemon Sapphire on my GBA!)
As for the water move, I usually like to use Surf, but based on the Type Coverage Tool, the following move types are best if you want to cover a lot of Pokemon's Weakness(es): Rock/(Flying covers more, but that's impossible for Swampert), Ice, Fighting, & Ground - Type Moves.
I have 2 Move recomendations for each Type. They are divided between whether the moves make contact with the opposing pokemon or not, except the Fighting-Type Moves, since they both make contact.
Ice = Ice Punch/Icebeam (whichever's your favorite or prefered move)
Rock = Rock Slide/Stone Edge (whichever's your favorite or prefered move)
Fighting = Brick Break/Hammer Arm (whichever's your favorite or prefered move)
Ground = Earthquake/Bulldoze (for those who want a physical (contact) ground move - I think it makes Physical Contact)
For those who still want to use a Water Move, You have two choices to choose from, but I recommended the following moves depending on which attack you have for your ground move. Though your free to use both Earthquake and Surf at the same time. Personally, I usually use Surf, but Waterfall may be good too. I never used waterfall with Swampert before, partly/mostly because the battle(s) usually ends with just one attack, (Well, most of the time...), so I don't know how well Waterfall works with Swampert.
Water = Surf (with Bulldoze) / Waterfall (with Earthquake)
Also, if your going to make room for a water move, I'd suggest replacing the Ground-Type Move with the Water-Type Move as you would cover more pokemon that way than if you had replaced the Fighting-Type Move instead.
This is really a matter of Personal Opinion and Preferences/Prefered Choices...