Here's some of the pranks that are played by senior sailors on newbies. Even if young sailors have doubts, it's hard to question a senior sailor's orders when still fresh out of training. You may want to warn your sailors about these.
Mail buoy watch
A young sailor is given a large hook and binoculars, fitted with a harness and safety line and stationed on the weather decks near the bow. S/he is told s/he on mail buoy watch. The imaginary "mail buoy" contains all of the ship's mail, dropped off by another ship earlier that day. Instructions are given that when s/he sees a buoy of X description s/he must hook the buoy or the bag on the buoy. They're warned that the whole ship is waiting for their mail, and failure to hook it will have the whole ship mad at him/her Sometimes even officers are in on the prank.
This can work to the sailor's advantage, particularly if the sailor's job is not terribly pleasant. I knew one young Fireman who was assigned to the boiler room who spent many hours on mail buoy watch. When another sailor finally "took pity" on him, he owned up to knowing about the prank, but the prank assignment was more pleasant than his regular job.
Junior sailor is assigned to watch sinks and toilets in the head (bathroom) for any leaks as the ship switches systems before getting underway.
Go get a:
A young sailor is told to go get three feet of gig line. "Line" in the Navy means rope. S/he runs around the ship asking where a gig line is. Most senior sailors know about the prank and will continue sending the young sailor on to someone else as soon as the question is asked. Either the sailor figures out for him/herself what's going on, or a kind senior sailor explains that a "gig line" is the line a button-down shirt, belt buckle and pants zipper flap make down a person's front. Other versions: flight line, shore line, chow line.
Bucket of prop wash
Usually on an aircraft carrier, the young sailor is told to get some prop wash to wash planes. Like the gig line prank, s/he runs around asking for it. Prop wash is the wind that comes off of an aircraft's props or jet engine.
A young sailor is asked to go to the boiler room for a BT punch. A BT is a boiler tech - and sometimes happy to oblige. Other versions are left-handed punch or right-handed punch.
A sailor is told to go to stores to get a long weight. The SK (storekeeper) goes away for a while to "look" for it. On his return he says "was that long enough?
A water hammer is the banging sound pipes sometimes make
Can of water slug
On submarines, a water slug is the water that is ejected when a torpedo tube is fired without a torpedo in it.
Binnacle Alignment Tool
A binnacle is a box or stand where, on old ships, a compass was kept. It does not exist on modern ships. It is also a side branch of a river.
Darken the chief's mess
Young sailor is sent to the Chiefs' mess to turn off their lights when "darken ship" is piped. Darken ship only applies to lights that can be seen outside of the ship. The chiefs do not take kindly to their lights being turned off.
Young sailor is sent to the galley to get some oil boiled. Linseed oil is "double boiled" so it is sent back with the sailor saying "you've only boiled this once"
Shore line stretcher
Another one for the really gullible. Shore line is where the ocean meets the land, the line of beaches and/or cliffs.
A sailor is told to go get a left-handed screwdriver, pliers, etc. It is always a tool that can easily be used in either hand.
Green oil for the starboard lamp/red oil for the port lamp
Naval ships have not used oil lamps in a VERY long time, and the color comes from colored glass, not colored oil. Only a very gullible sailor falls for this one. Also, Pink paraffin for the night lights .
Sound powered phone batteries
A sailor is told to get a recharger or batteries for the sound-powered phones. Um... SOUND-POWERED phones, anyone?
A young sailor is sent to: Engineering Control to report, "Sir, High level alarm in the cooling system, request permission to blow the MPA". Translation? Oral sex for the main propulsion assistant.
A sailor is told to climb through (or clean, or whatever) the fallopian tubes. Of course, fallopian tubes are part of the female reproduction system.
It doesn't work as well with a better educated Navy and so many female sailors around, but it sometimes still happens. Star Trek fans, accustomed to hearing about the Jeffreies Tubes, long access tunnels that run through the Starship Enterprise, are especially prone to falling for this prank. Sometimes also confused with an old-fashioned "speaking tube."
I'm sure there are other pranks I'm not familiar with, but it's a start.