The nautical phrase different ships, different long splices ¹, and its variants, are used figuratively to mean that different countries (or cities, spheres of activity, etc.) have different customs or practices.
—Cf. also different strokes for different folks.

¹ The nautical term long splice denotes a splice in which the ends of two ropes are interwoven in such a way that the point of joining and the ropes are of equal thickness.

These are, in chronological order, the earliest occurrences of the phrase different ships, different long splices, and variants, that I have found:

1-: From At Sea with Joseph Conrad ² (Boston and New York: Houghton Mifflin Company, 1922), by the Royal-Navy officer John Georgeson Sutherland (1871–?)—the author writes that the British naval officers, although delighted with the prompt appearance of U.S. destroyers in European waters after the United States had declared war on Germany, were surprised by the lack of formality on the U.S. vessels:

Another commander had just completed adjusting the compass of an American destroyer when he politely asked the lieutenant in charge if he would kindly lend him a pair of binoculars. The lieutenant shouted down the forward hatchway, “Anybody down there?” Back came the answer, “Yep.” “Well, say,” continued the lieutenant, “one of you go down to my cabin and in the middle drawer on the right-hand side you will find a pair of binoculars; bring ’em right along”; the reply to which was, “It shall be done just exactly as you say, lieut.” One can imagine the difference on board a British warship, where a bluejacket would have bounced up a ladder two steps at a time and on reaching the top would have sprung smartly to attention, saluted, and with an “Aye, aye, sir,” carried out his instructions. There’s an old saying, “Different ships, different long splices,” and I suppose it is the same with nationalities, “Different countries, different customs.”

² During the First World War, John Georgeson Sutherland was the captain of the Q-ship Ready, in which the Polish-born British novelist Joseph Conrad (Józef Teodor Konrad Korzeniowski – 1857-1924) made a perilous ten-day voyage in November 1916. Sutherland capitalised on Conrad’s fame by spinning out the experience in At Sea with Joseph Conrad.

2-: From The China Express and Telegraph (London, England) of Thursday 24^th July 1924:

ADMIRAL LEVESON ON AFFAIRS IN CHINA.

Admiral Sir Arthur Leveson, K.C.B., Commander-in-Chief of the British China Station, was the guest of honour at an Anglo-American Association’s tiffin at the Grand Hotel des Wagon-Lits, Peking, on May 23 […]. Admiral Leveson said he should like to congratulate the civilians on what he had seen out here in the way of the advancement of civilisation and the significant work that all the various civil elements were carrying out under difficult circumstances all over the Far East. One of the impressions he had was significant—shipping—the size of the vessels of all sorts and of all nations and their numbers. Wherever one went there he saw numbers of these great vessels, and in no other part of the world were so many ships of such size and efficiency to be seen.
Another thing which Admiral Leveson said had impressed him a good deal was the way in which tropical races out here were being administered by the more civilised peoples, as in the case of the Dutch in Java, the French in Annam, the British in Malaya, the Americans in the Philippines. He thought it was perfectly wonderful to see all these different nationalities handling the situation in the way they were handling it. There was an old nautical adage that “different ships have different long splices, but the splice holds nevertheless.”

3-: From the Tampa Morning Tribune (Tampa, Florida, USA) of Thursday 9^th February 1928:

Henry Ford Takes Ride in Millionth Flivver

PEB: “Tavares man eats millionth orange,” says The Tribune. After our slide-rule had cooled down from estimating the daily average for a hypothetical epicure of 60 years’ perfect digestion, some important statistics were derived from the computations.
After much deliberation, we can only say “Different ships, different long splices,” but figures don’t lie if slip-sticks don’t slip, and THAT million oranges, placed in a line, or on a truck, or in a packing house, or as you wish, would stretch . . . at least to a point 10 miles from verity.
PROF. HAMMOND EGGER.
Clermont.

4-: From a letter to the Editor, published in The Londonderry Sentinel (Derry, County Derry, Northern Ireland) of Thursday 12^th April 1934:

Dear Sir—Last Monday the Londonderry Corporation refused the offer of the Leicestershire Regimental Band to play on Sundays in Brooke Park. On the same day the following appeared in the Press:—
“The bands of the Life Guards and Grenadier Guards, by command of the King, played on the east terrace of Windsor Castle yesterday afternoon, and the public were admitted. Several thousand visitors listened to the music, and during the afternoon caught a glimpse of the King and Queen when they came to the windows of the Royal apartments.”
There is an old saying at sea—“Different ships, different long splices.” Enough said!—l remain,
F. GILLILAND,
Commander, R.N.V.R.
Brook Hall, Londonderry.

5-: From the Liverpool Echo (Liverpool, Lancashire, England) of Friday 1^st April 1938:

Old Shell-Backs And Chanty Men
Nautical Phrases Of The Past; Blow The Man Down; “Bare Whack” Ships
By CAPT. J. G. BISSET, R.D., R.N.R.

With the passing of the sailing ship, many quaint old nautical words and phrases have disappeared from the seaman’s vocabulary. Old timers who have served in sail occasionally revive some of these full-flavoured expressions when spinning a yarn, but to the modern seaman, brought up in “steamboats,” they have little, or no meaning. […]
[…]
Being inveterate grumblers—or grousers—the old “shells” always spoke of their “last ship” as being a paragon of all the virtues as compared with the present one. Any slight difference of routine between two ships was lugubriously summed up as “different ships, different long splices,” but as long splices NEVER differed, what they really meant was, that from the point of view of faults and “awkwardity”—a favourite term, pronounced “awkquidity”—all ships were about the same—left-handed, three-cornered, soul-searing, back-breaking. cock-billed, half-starved, undermanned work-houses.

6-: From the column Soundings, by Casey Davison, published in The Tacoma Times (Tacoma, Washington, USA) of Saturday 17^th February 1940:

PAGE MR. GLENCANNON

“Different ships—different mates and different long-splices.” So they say.
Out in Honolulu’s harbor sputtered Capt. Gerasmios Panas as he stomped the bridge of the Greek freighter CRISTOS MARKETTOS this week.
He’d just bunkered his ship with Pearl Harbor coal. He’d paid a good price. Capt. Panas burned. The coal wouldn’t.
Uncle Sam, a good merchant, sent a crew of Samoan firemen aboard the CRISTOS MARKETTOS to demonstrate navy methods. The boilers heated. Capt. Panas cooled. The CRISTOS MARKETTOS plowed slowly to and fro in the harbor. The navy firemen went ashore. The freighter left for Japan. All was well.

7-: From The San Diego Union (San Diego, California, USA) of Sunday 10^th November 1940:

Along the Embarcadero
British Skipper’s Brilliant Plan Miscarried; Nosey Lieutenant Was ‘Villain of the Piece’
Attempt at Speed Brought Praise—And Much Trouble
By Jerry MacMullen

Different ships, different long splices—so the saying goes—and also, different skippers.
This holds true for admirals as well. Some are demons on gunnery, some lean toward extra proficiency in engineering or in communications. Some have strong tendencies toward collision-mat ³ drills, while others are fond of a minor evolution which comes under the head of “Away, fire and rescue party!”

³ The term collision mat denotes a large mat for covering a hole in a ship’s side, in case of a collision.