Hi sean.

Well to help you out a bit,i just buy this great VIIC U-Boot from Revell myself and just finished working on the two halfs of the hull. In general you can open up all the flood holes,what i did was using a tool like dremmel has,grinding of from the inside of the hull parts and then using a lamp to see were the food holes are in general you can spot them very easy,in this way you also create the look like the thin outer hull effect,but be warned grinding and drilling all those flood holes takes alot of work( i think its more then 147 or so) but i did it! and must say it sure makes alot if differents,Because in this scale it just look so much more realistic and it was worth every single minute of work! One tip there: you need a good tools and a steady hand ;-)
Some more things you should considering about to make your u-boot looks so much more autentic are the wooden deck panels from Nautilus Models, and this great Photo etched bras set from Eduard! I ordered them too and just today i got the wooden deck panels from Nautilus Models and WoW! every single flood hole is open and it fits like a glove! its worth very single penny! The Eduard Photo Etched Part is doing the rest! Well to thing about the pressure hull there is a kit you can order that have something like that but hey!...Realy this can be made easy by yourself,all you need is some isolation pipes from the do it youself shop,in general you need only the fore and after section to fill up because thats were the bigger flooding holes are,the great thing about using isolation pipes instead if real plastic pipes is they can be shaped into these thight section of the hull and trust me,ones you fit the deck panels you not see much of this pressure hull so to much extra work here is a bit useless,the main thing about putting something there that look like there,s a pressure hull for me was,when you took the hull with deck panel fitted that you could see true this flood holes and see the other side of the hull,i did some testing with thos plastic pipes you can buy for use normaly in house-building to connect water pipes and stuff but then you would need a bigger section for the middle,and two smaller sections for the fore and after section,but the piont is you not even anything from this middle section because those holes in the middle section are so small you not see anything there from the pressure hull. So what i did was using those isolation pipes cutting them in the right lengtes and putting one in the fore section (read torpedo section) and another in the after section because of the fact the are bent easy and can be fitted almost everywere they a great thing here! And after i fitted then and glued the deck panel on it just looks amazing because now it ealy seems like there is a pressure hull behind those flood holes :-)
So i hope this helps you out a bit.
Enjoy Building this great fantastic kit!

Richard