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

A similar sentiment was expressed by a Mrs Cornish of Eton, apparently (about whom I can find nothing but that she was a generation earlier than the novelist A.C. Benson, 1862-1925). When under the stresses of life she is reported to have drawn herself up to her full height and consoled herself with the reflection that

I am an Englishwoman;
I was born in wedlock;
I am on dry land.

Source for the name of Mrs Cornish: http://www.johnsandoe.com/review_2161.htm. I have known the expression for a long time, but this is the only reference to it I can find on the Web.