Perhaps Zaat (also known as The Blood Waters of Dr. Z) 1971?
Still not B&W, but plot points match....
From the wikipedia summary:
The film begins with Nazi mad scientist Dr. Kurt Leopold in his lab,
where he has lived alone for about 20 years. His formula (the titular
Zaat) can turn a man into a walking catfish. He injects himself with
the serum and emerges from a tank as a giant fish-like creature.
Leopold decides to kill the colleagues that laughed at his work. He
begins with a character named Maxson. In a lake where Maxson is
fishing, Leopold swims under Maxson's boat, overturns it, and proceeds
to kill Maxson and Maxson's son. Maxson's wife escapes, although she
is in shock from the attack.
After killing Maxson, Leopold discovers a girl who is camping out
alone on the shore of the lake. Leopold rlater eturns to the lake
where the girl is still camping and waits for an opportunity to abduct
her. Lopold catches her underwater. He swims with her to his lab, even
as she struggles in vain to escape. Leopold injects Zaat into her
neck. As she is immersed into a tank of Zaat, the girl wakes up and
struggles against the ropes holding her. The equipment malfunctions
for reasons unknown, and her corpse, partially transformed, is pulled
from the tank.
Leopold attempts to kidnap another mate. His choice is Martha Walsh,
the lovely female member of the INPIT scientific team sent to
investigate the weird happenings in the town (caused by Dr. Leopold).
Leopold grabs her after her male counterparts leave her alone. Leopold
takes her to his lab, but two of her companions (having unraveled the
plot) are waiting there. Leopold kills them (including the sheriff)
violently. He injects her with Zaat, readies her to be dunked into the
tank, and makes his getaway, with canisters of Zaat. Martha's
transformation does not go as planned and she gets saved from getting
dunked in the tank by one of her dying male companions as Leopold
flees toward the ocean. Despite being saved from the transformation,
she appears to be in a trance and immediately follows Leopold into the
sea. The movie ends ambiguously, with Leopold seen shot but not