A few tips that might be helpful when solving Printer's Devilry clues:
1. Forget all about normal cryptic clues. You are trying to slot the consecutive letters that make up a real word into the clue as written - there is no 'wordplay' and the meaning of the inserted word has no relevance whatsoever.
2. Whilst the 'devilled' (printed) version may resemble a cryptic clue (eg "As soon as the teacher's goon is finished (9)"), and might lack elements required to make a sentence, the undevilled form is always a 'real' English sentence ("As soon as the teacher's goNE THE LESSon is finished"). [The solution here is NETHELESS]
3. The breaks (at least in Azed PD puzzles) always appear part way through a word in the devilled clue (eg 'go/on' in the example above).
4. This means that there is often a word that stands out (as 'goon' above) as the location of the omitted letters. If letters had been omitted from within 'finished', say, in the example above, that would leave the unnatural-sounding "teacher's goon is" intact, and even with changes of spacing and punctuation this won't work.
5. The sentence that is revealed after reinstating the missing letters will not contain any superfluous detail, so "As soon as the Latin teacher's goon is finished" would be unfair (and unacceptable) as a PD clue for NETHELESS since the addition of the word 'Latin' serves only to take the solver in the wrong direction.
Based on the foregoing, in the example "Cruise passengers often like tower's decks (7)", we are immediately suspicious of "tower's" - the word sticks out like a sore thumb and must surely be the place where text has been lost. This means that 'cruise', 'passengers' and 'decks' are all there to provide context, clearly a nautical one.
The clue could potentially have been written without the word 'cruise' (ie "Passengers often like tower's decks"), so it must be there to guide us towards the sort of ships that we would associate with cruises, ie 'liners'. Let's assume that the break is in "tower's" and that 'liners' are present in the undevilled version. The letters 'ers' appear at the end of "tower's", so we are surely looking at:
"Cruise passengers often like tow ???? LINer's decks"
Rule 2 says we must have a proper sentence, so something like "I am emplyed to sweep liner's decks" is 'crosswordese' and would be no good; to be a valid sentence there needs to be an article ('a' or 'the') before 'liner'. So
"Cruise passengers often like tow ??? A LINer's decks"
We ask ourselves what passengers specifically do on the decks that they wouldn't do in their cabins. Probably get some form of exercise, perhaps a walk...the last piece of the jigsaw which makes the undevilled sentence
"Cruise passengers often like to wALK A LINer's decks"
and the solution ALKALIN.