An Hospital ship is designated as a Type13=Auxiliary Ship in fotrs and would display a blue sonar line, that should make it a valid target to attack me thinks. Can Mr beanie confirm if this is the case and its definitely classed as a Type13 and its not a typo, if it is true then you've stuck 2 fingers up to the Geneva conventions rules of engagement wotsit.
[UnitClass]
ClassName=NSHOS
UnitType=13
AppearanceDate=19180101
DisappearanceDate=19470101
DisplayName=RC Hospital Transport
[Texture 1]
TextureName=data/Sea/NSHOS/NSHOS_T01.tga
LightmapTextureName=
NormalmapTextureName=
StartDate=19000101
EndDate=19991231
Frequency=1
[Unit 1]
Name=Hospital Transport
DOC=19180101
DOD=19470101