Dear Meg,
A similar painting to the one you describe of a man on a raft being
attacked by sharks is "Watson and the Shark" in which Watson went swimming
in Havana Harbor from a merchant ship and was attacked by a shark which
took part of one leg. Watson went on to Mayor of London ( I think). The
painting is in the collections of the MFA in Boston, check their web site
for more info. on this wonderful painting. I thought I would pass this
additional reference on in case you wanted to explore other seascapes
infested with sharks.

Sam Shogren
Curator, Penobscot Marine Museum, Searsport, Maine