I don´t dispute the need of being prepared. But I think that in this particular situation other means might be better. If someone is in the same flat a whistle could be a little overkill. If nobody is in hearing range the whistle is inadequate. A cell phone in a waterproof pouch could do more (I have that pouch for my phone because on rare occasions I might take a bath in a beautifull river or lake while still fully dressed <img src="images/graemlins/frown.gif" alt="" />).
I know a couple of these signal devices. So far I didn´t encounter one that was submergible. So maybe taking it into the shower will destroy it.