Learn English – Passive voice for sentences using modals and “used to:

Andrew Leach's explanation is sound as far as how you might go about doing this, but the answers all sound awkward and goofy. I think this points to a bigger point that you shouldn't do this.

Consider the reasons for using passive voice. The whole idea of passive voice is inverting the emphasis between subject and object, so it just doesn't make a whole lot of sense to do in imperative sentences.

I've actually already written about this on ELL. My conclusion has been that all style guides fail to adequately classify sentences like this in their two category system of active vs. passive voice.

This is why linguistics breaks things down into more than just active and passive. "Middle voice" is just one of the many terms out there that describe this, according to Wikipedia:

Patientive (S = O) ambitransitives are those where the single argument of the intransitive (S) corresponds to the object (O) of the transitive. For example, in the sentence John (S) tripped and John (A) tripped Mary (O), John is not the person doing the falling in both sentences. Likely candidates for this type of ambitransitive are verbs that affect an agent spontaneously, or those that can be engineered by an agent. English has bend, break, burn, burst, change, cool, enter, extend, fall, frighten, grow, hurry, melt, move, open, spill, stretch, trip, turn, twist, walk, and many other verbs.

Verbs of this class have been called unaccusative verbs, middle voice, ergative or anticausative verbs in the literature, but again, these terms are not universally defined.

Also to answer your titular question, this is unmistakably passive voice without be:

The ship got sunk.