Decals to use for the wooden deck of my Bismarck?????

It could work, but investigate staggering the ends of the planks on your square of printed decal the way you did with the middle.  You'll end up with a jagged square in the E/W direction.   Mate the next decal run with the jagged end of the previous decal.   You don't want the ends of a number of planks to end on the same line. 

Also, consider how you will approach the fill-in between two fixed length endpoints.  Will you apply the middle then cut each end (with a jaggie) to match a bulkead patterns.    Or will you cut and apply the end pieces and have to go back with a short jaggie piece to fill-in the middle.

Paint the deck your predominant decal color before beginning so you don't have unnatural colors showing through gaps.

While the time required to cut and apply masks to the deck to paint it with multiple coats may take a while to get done (mask/paint/dry repeat)

will the time and effort be offset by the repeated application of decals, setting solution, and allowing to dry?

 

   It's possible. You would be best off to paint the deck one of the colors in the decal, with a gloss finish (I don't know what color plastic they used for the deck), and decals prefer a gloss finish, especially if you need to make the decal film "disappear". Setting solution (Microset, or Solvaset, etc.) will help get the decal to settle around any details on deck. I've never tried this so don't know how it will turn out visually, but it sounds like a reasonable way to reproduce the deck appearance, in this scale.