Martin

A water filled bunker is an abnormal ground condition. The rule you need to look at is 25-1c.

The first few lines say,
"It is a question of fact whether a ball lost after having been struck toward an abnormal ground condition is lost in such condition. In order to treat the ball as lost in the abnormal ground condition, there must be reasonable evidence to that effect. In the absence of such evidence, the ball must be treated as a lost ball and Rule 27 applies.

So if you can say with confidence that the ball is in the water filled bunker you may take relief by dropping a ball outside the bunker according to Rule 25-1c(ii). This will cost you a penalty stroke.

Best to read the rule thoroughly yourself. You can read it online at www.randa.org if you don't have a paper copy of the rule book.

Of course get back with any further questions.