There are some backpacks in the game that give you more item slots. If for some reason you miss them, after the wrecked ship area, you will swim through a swamp. Once you climb the ladder, you will find a backpack sitting there. This only appears if you missed one.

Backpack 1: While inside the old house, you'll come to an area that's outside. There is a shack that has the backpack inside

Backpack 2: After obtaining the snake key, you will go into a master bedroom with a clock that you must set the time for in order to move a bed. The backpack is in the walk in closet.

Backpack 3: Floor 1 of wrecked ship, inside the Recreation Room. You will need to have a corrosive in hand