AC 135-42 provides regulatory guidance if you are indeed flying a 135 ETOPS mission.

Regulation aside, I would treat it (depresurization) like it was going to happen; and at a minimum, don't dispatch unless you have at least VFR weather forecast/fuel minimums at your ETP alternate(s). Some arguments can be made that you can stay up higher (i.e. FL250) and suck on oxygen to extend range, but many oxygen masks/systems have the caveat of "extended use at cabin altitudes above FLXXX not recomended."

Brainstorm and run worst-case scenarios through your SMS and with your dispatcher and compare it to the UV flight plans before you commit to your customer on an over water (or over Somalia) trip.

My favorite scenario is the cracked inner pane on the windshield that requires dumping the cabin and a descent. Once you descend, the window totally delaminates and sends chunks of polyurethane through an engine, requiring a shutdown.

(E)ngines (T)urn (O)r (P)eople (S)wim, right?

If you can land at one of your ETP alternates that has a VFR weather forecast with VFR fuel with this scenario, then give it a shot.

This scenario might sound overly pessimistic, but that is what a process approach to safety is all about.