frostweaver said:Swampert alone cannot learn Hydro Pump. You need to lvl up Mudkip as a Mudkip until it learns Hydro Pump, then evolve it. Use heartscale to bring back moves such as earthquake. Right now in your situation Swampert cannot possibly learn hydro pump, sorry!
frostweaver said:Just give Mudkip the everstone to hold on to (or hit the B button whenever you see the evolution screen) until it learns Hydro Pump. The game gives you a chance to evolve every level after you met the basic requirement level. So say you declined evolution at lvl 16 and you lvled up to lvl 17. It'll try to evolve again. Decline it. And then at lvl 18 it'll try to evolve again and so on.