There isn't a U/W casing i could find for FZ18..... not by Panasonic, not by 3 party. So that very much ends the 'How Much' part.....
2nd......
You need 'flash' for macro underwater.... really really important. But not for anything more than a few metres from you if you are using the in-built flash. All you will see is a lot of 'spot' infront of you subject..... case by you flash reflecting all those tiny stuff floating in the water......
We use 'U/W strobe' (aka.... U/W flash) for taking wide angel shots.