From 1937...

Wealthy men are disappearing. They've all been involved in legal
swindling. So when Monk Mayfair takes Ham Brooks for three million
bucks, he becomes the latest target of ... The Sea Angel!

Of course, the Sea Angel is just a guy in a funky sea monster suit.
Everybody says this, from his first appearence. But he can't be
harmed, and he gives off electrical shocks, and he can fly and swim
very fast...

Eventually the plot involves a World War German U-boat and a quarry on
the coast of labrador where the 22 kidnapped financiers are being
held. there are two competing criminal gangs to sort out/ Oddly, the
bad guys are

SPOILER ALERT!!!!!111

using the Sea Angel (it is just a ridiculous costume) to try and
reform the real bad guys (swindling rich dudes) by scaring them to
death. a susposedly "reformed" rich dude tries to horn in on the
action with a kidnapping scheme. DOC and the "Sex Angel" gang leader
discuss approaches to criminology, and the guy agrees DOC is correct.

In this one, Ham supposedly goes insane when Monk swindles him. As
proof of his madness, he claims (in a November issue) that the
Brooklyn Dodger had won the World Series. The Dodgers finished a
dismal 6th -- 1937 was the second straight NY-NY series, again won by
the Yankees.

The blurb on the bak of the paperback harkens to a villian called THE
SILVER OGRE. No where did that come from>

Neat touch -- at the quarry, the swindlers are guarded by men wearing
green devil costumes