This takes place in the 19th century and I think part of it is in the Sargasso Sea. The most memorable thing about the movie is people “walking” on the sea while wearing hydrogen balloons to lighten their weight so they can walk on the Sargasso weeds. (Yeah, and 3 hydrogen balloons would give you about what - a half-pound of lift?)
I always thought this movie was one of those 3 “forgotten world/land/people films” that Doug McClure made:
Land That Time Forgot (1975)
At The Earth Core (1976)
The People That Time Forgot (1977)
but now I know it is not one of those 3 cinematic epics.
So, what is this Sargasso Sea movie?
Are you thinking of The Lost Continent?
That movie creeped me out when I first saw it. 
I also noticed this at the imdb link mentioned above:
Why was it banned in Finland?
Curse you, sad smiley! I didn’t put you there!
If the movie has any kind of anti-Russian message in it, that might be why. A lot of stuff got banned in here back in the day for that reason (or so I’ve heard).
Thank you Anamorphic and Nut Wrench
I was unsuccessful in my IMDB search.
Incidentally, there were 2 other movies that qualify for having The Lost Continent title:
Lost Continent (1951)
http://us.imdb.com/Title?0043757
and
Atlantis, the Lost Continent
http://us.imdb.com/Title?0054642