The sensors on my 350 CBR for high bilge water look something like this:
View attachment 9088
They are thin panels and when enough water gets under the flap, it triggers the alarm.
Location is on the stringer in the engine room, and then there's one near the shower/cabin sump area as well.
This past summer, the alarm went off, even though the bilge was dry. Formula replaced it, and let me know that from time to time, these sensors can short out (and thus sound the alarm...which, btw, will let you and the rest of the world know you're sinking).