There is no TM for Hydro Pump, unfortunately. Pokemon can only learn it by level-up or by breeding.
Your Swampert will never be able to learn Hydro Pump. Only Mudkip learns Hydro Pump, so if you really want a Swampert with the attack you'd have to breed to get a new baby Mudkip, then level the Mudkip (without letting him evolve) to level 42. He'll learn Hydro Pump, then you can let him evolve to a Swampert. If you don't want to breed, and depending on what level your Swampert is at I don't blame you, then you'll just have to stick with Surf/Muddy Water for a strong water attack.
As for Azumarill (I assume this is an Azumarill, and not the baby form of Marill name Azurill), simply level him up to 57 and he'll learn Hydro Pump.