I would surface and run full speed to close with the hospital ship to see if you take small arms fire... try to keep the hospital ship between you and the other destroyer if you can. This way if you start taking fire from the hospital ship you can dump 2-3 fish from 500-800m and then crash dive before the destroyer catches up. This way you won't have any morality issues...
BTW, a cross painted on there is supposed to mean it's protected, but at the same time the Geneva conventions weren't until after WWII...