Swampert is better because it has swift swim, is immune to electric, and uses stealth rock. It can also use earthquake to hit electric Pokemon and sludge wave to hit Tapu Bulu.
Swampert is better . Let's have a closer look. Blastoise weak to electricity. Too bulky . Weak to grass. To many weakness. But it's a good sweeper. Swampert a good sweeper less weaknesses. So my choise is swampert.