Ehh... what's the diff between a 50 cal and a 20mm cannon when you're in a submarine on the surface?...
- sorry, mark!
Now, old un-reliable wikipedia says
Armament- 3 .30 cal (7.62 mm) machine guns (two in nose turret, one in ventral hatch at tail)
- 2 .50 cal (12.7 mm) machine guns (one in each waist blister)
- 4,000 lb (1,814 kg) of bombs or depth charges; torpedo racks were also available
at
https://en.wikipedia.org/wiki/Consolidated_PBY_Catalina
Dependent upon the mod, your boat might be very thin-skinned also...