Dropped my nokia in a pint of wine at christmas and it packed in for a few weeks then started working again, then after a few more weeks packed in again and it's been working perfectly now for about 7 months.
If it was just water, it should work perfectly well once dried out - I was told that the reason mine played up after I'd dried it out was that it was the wine causing the problems and that I should have washed it off thoroughly in clean water!!
Take the battery out, sim card and any covers off and leave them on a warm radiator for 24 hours, turning occasionally to allow any trapped moisture to be evaporated.
Don't know about tesco, but all asda's phones are unlocked