1850s. When a shipwreck survivor swaps his convict uniform with a drowned lieutenant, he must keep up the charade or face execution from his rescuers.

This has the potential to describe an interesting period piece that can be set in any newly colonized land however the plot is vague and makes the story seam weak.

Better to define the shipwreck as the inciting incident then a corresponding goal.
What is it the convict wants? To get a plot of land in the new colony? To kill the officer that lashed him on the ship? To get revenge against the people who sent him away from home?

For example:

After a shipwreck a surviving convict takes the clothes and assumes the identity of a drowned lieutenant he works his way up the military ranks in the new colony so he can kill the commandant who condemned him to exile.

Hope this helps.

the expression "keep up the charade" is too vague: it's what all the film is about!