Do you remember the study & the secret door, opened by the books beuing put in the right places? Open that, get past the safe , turn the lights on & then down the stairs to the lagoon. Damaged one man sub is in the lagoon.
Go to Study. Take all the books that are either lying loose or that you can grab from the shelves. Replace red books in order of shade (dark red, light red, dark red) on shelves. Secret Door opens. There's a safe - combine Marston's note, decoding letter & Emily's Bible, for combination to safe. Open safe. Small room. Locked door. Deflated dinghy, & series of boxes, which hold things & is also an anagram of the sign. Solve anagram, door opens, turn lights on, go down stairs & observe lagoon