When Bombard Ketch comes on board, deal 3 damage to a base an opponent controls.
22
Explained
Stowaway 2802.4
When you scrap this card, you may pay {2}{CO} to deploy it instead.
When Bombard Ketch comes on board, deal 3 damage to a base an opponent controls.301.10
Not when you deploy it: it happens once it has resolved and is on board, after your opponent's chance to react. Put on board without being deployed, as by Resurface, it happens too. In the sortie phase it goes into the crossfire (301.10). Choose one of your opponent's bases on board when it happens; with only one, it is chosen, and with none, nothing happens. It takes the damage, which stays; a base left with no hull is destroyed at once (705.7c).