I know the path through the bilge and lab are considerably easier than the cafeteria and bridge. Promenade deck is second hardest if you don't grab the key underwater and head back through the casino and slip another key from the observation tower. You can just ignore all enemies which saves tons of ammo.
Yes there is....weapon case is located in the lab.....also it is the quickest path due to all low level monsters until you hit the side of the ship immediateely before the deck. be careful there though, on the side bridge you see one scagdead infront of you, but another spawns behind you.