@UmbraLux:
Belial's character in my PbP game rarely casts a non-offensive evocation, because he optimized for offensive power. His character can end most fights in one shot. Which is hard to GM for, but I've managed decently so far.
Perhaps because he min/maxed his casting? Or because you haven't put him on the defensive? Shrug, I'm just throwing out guesses. It does surprise me you'd want to make the magic he's not using just as (or nearly as) powerful as his attacks - particularly since you say it's difficult to GM.
My RL game does not have a spellcaster, though it's sort of supposed to.
So my limited experience suggests that it's quite possible to use only offence if you really try. I don't know if the same is true for defence.
It's certainly possible. It's possible to treat everything like a nail if you only have a hammer.
Jokes aside, some situations make attacking a suboptimal choice at best. Whether or not those get used depends on the group. The thaumaturgist throwing weapon 6 "potions" around is the "optimized" character in the game we're playing.
My point about underwear foci is that unless the GM is out to disable you then you don't really need to worry too much about losing access to foci. And if your GM is out to disable you, he can do that anyway.
(Anyway, why would wetness disable underpants foci?)
The "GM is out to disable you" characterization is simplistic at best. Have you tried carrying a staff around the city? Into the White House? (Our game is set in DC.) Or even city hall? What about the police station? A neutral's house? There are lots of potential reasons for not having access to foci.
Edit: And water disrupts magic so might be compelled to temporarily disable foci which soak it up.