Das Boot had an electrical fire during the time they were being DCed after attacking the convoy if I remember rightly.
Would be interesting to have a fire break out and have it knock out certain systems like the TDC or have it so you can't emergency surface, something like that maybe?
Not have an actual fire, but just simulate it's effects. It'd probably be best to have a message come up.
"Fire in the **boat compartment**!"
"**Ship system** Has been damaged!"
I suppose you can treat it like an isolated flooding...And the fire would eat up oxygen.
That could make things hairier in an already hairy situation.