Some folks use a rubber-based cement to hold the deck into place. I prefer to rely on mechanical means to keep the deck in place....especially since the metal deck will expand/contract at a different rate than the underlying plastic.
For my Type VII's I use a number of things to hold the deck sections in place....all items that are normally seen on the deck. No exposed screw heads and such for me!
We're talking about things like the bollards, sound head, fairleads, deck gun and hatches. Also don't forget that the conning tower can be used to hold part down too.
The December issue of the SCR will include an article that I am writing that details how I built the OTW Type VII....but sections like attaching the deck apply to any U-boat.
-hope this helps,