They carried many combinations of weapons and depth charges were one configuration.
http://eng.pt-boats.net/museum/museum_3.html
http://www.users.zetnet.co.uk/mbriscoe/PAGES/Boats.htm
http://en.wikipedia.org/wiki/Motor_torpedo_boat
They will occassionally drop on your last know location. So for 99% of the time they will be a very poor platform.
But for the times that your periscope is sighted because you just had to have a peek.... :rotfl: