Anyone know what type of boat this is on?

Not much to go on. Looks like a 3" and since it's on a pedestal rather than
a turret, I would say it might be mounted on a WWII submarine.

WWII ship mounted 3" would have been on a roller path and the gunners would
have rode in seats and cranked it around the roller path, along with the gun
shield.

Also it looks like it has range finder type sights, which would would also
be WWII vintage.

I believe, what we are looking at, is the gun is train to the side of the
boat since I can see the curvature of the hull in the lower right picture.

If you could look at some of the WWII subs in an old Jane's Fighting Ships,
you could determine which ones had a deck mounted artilary gun.

Just speculation, FWIW.

Steve
s/v Good Intentions