There is currently no feature-length movie of “The Secret Sharer” commercially available, but you can watch a television length one on-line.
There are some productions of “The Secret Sharer” available. For example, “"The Secret Sharer" was adapted for film and produced by Encyclopaedia Brittanica Educational Corp. in 1973.” (enotes). You can find a link to it below.
There is currently a movie version of “The Secret Sharer” coming out this year because it has a 2013 release date. There is not much information about it, but it is a relatively low budget movie produced in Mandarin and English and Jack Laskey.
The Secret Sharer is a contemporary love story about two strangers who meet in unusual circumstances - on board a rusting cargo ship sailing its last voyage, to the scrapyards in Wenzhou, on the coast of mainland China. (imdb)
Other movie versions include a 1967 film and a television episode of the British series BBC Sunday-Night Theater. It was entitled “The Secret Sharer” and you might be able to find it if you subscribe to Netflix.
Unfortunately, you are not going to be able to find a solid movie adaptation of this story at this time.