You know there already is an alarm when you get breached.
#Sirens in ships when somebody boards!
fallen geode
safe sundial
Ratting away on someones ship is rather hilarious id hate if there was a constant noise that never turned off while a player was on a ship.
Though it would be funny if there was an alarm that a player them selves had to turn off, so the crew would have to clear the ship and manually turn off the alarm or the rat could turn off the alarm or the rat could turn the alarm back on... shenanigan's to be had