A British idiom, meaning "there are much worse things that this". Probably owing its origin to the (justified) reputation of the sea as a place of disasters, wrecks and death.I was not able to find a first date of use for this expression.
Native english speakers are offering another interpretation, that's to say "It is not that bad". This is a more optimistic interpretation, which reminds me of the Engineer's Consolation, namely
you can't fall off the floor