Shipwreck Back Floor
The Shipwreck back floor is really unique compared to the previous two dungeons. In order to reach this back floor you must buy (or find in the dungeon) a Flapping Fish. Oh and the other thing is (and here's the really annoying part) you'll need to keep Ice in your inventory in order to keep the fish fresh. The ice melt fasts, usually the time it takes to clear one floor of a dungeon is enough to melt the ice.
As you could imagine this makes it impossible to stock up on back floor keys for a long dungeon crawl. If you're trying to get to a back floor in Shipwreck you have to know what back floor you want to access before even stepping foot into the dungeon.
When you bring the flapping fish to the giant ? on the map it'll summon a fish who is wearing a captain's hat, he can be seen in the screenshot below. Give him the Flapping Fish and he'll eat your character and take them to the back floor. Also, as with all previous dungeons you can continue exiting/returning to grind ABS for a weapon if that's what you'd like.
If you wish to keep grinding the back floors in Shipwreck you won't need to keep bringing the Flapping Fish with you. Feeding the fish once will give you a free ride to the back floor until you advance floors or leave the dungeon. Something worth noting is after a lot of internet research I found that Shipwreck back floor 15 is one of the best locations to grind up your ABS in all of Shipwreck (and maybe even the game).