Aurora beam/Ice beam
If you want Shellder to evelove early choose aurora beam if late use ice beam.
Good water type move useful in game
You said that you would use him in game then I would choose whirlpool. Because it is great when you need in outside of battle but if you gonna use him to win over the E4 I would choose Blizzard
Powers up water type moves and Thunder Accuracy
Choose which of them you want Hydro pump for damage Surf for HM.
Great power 100% accuracy during rain
Hope this helps