The jocular nautical phrase mahogany reef denotes a bar, i.e., a counter in a pub, restaurant, etc., across which alcoholic drinks are served.
—Cf. also the phrase to amputate one’s mahogany.

The earliest occurrence of the phrase mahogany reef that I have found is from the following letter, published in the San Francisco Chronicle (San Francisco, California) of Thursday 17^th January 1924, i.e., during Prohibition—the author punned on bar in the sense of the legal profession:

Everywhere the Lawyer Went the Thirst Was Sure to Go

Editor The Chronicle—Sir: The editorial in today’s Chronicle, dealing with the American Bar Association’s proposed voyage on a British ship awakened fond recollections of some of my own “bar associations” in the wet and wicked past, when “schooners” crossed the bar with foaming regularity and it wasn’t considered criminal to “pilot” one or two across the mahogany reef.
No doubt the members of the association are all learned fellows who know “what’s what.” They know the law, and mean to abide by it. They have thirsts, even as you and I, and mean to assuage them. Therefore, in the only manner compatible to a law-abiding thirst, they will cross the bar, and, as becomes their noble profession, will again be “admitted to the bar.” More power to ’em.
SKIPPER.
San Francisco, Jan. 15, 1924.

The second-earliest occurrence of the phrase mahogany reef that I have found is from an article about men’s fashion, published in the Fort-Worth Star-Telegram (Fort Worth, Texas) of Tuesday 16^th March 1965:

Experienced sailors also know the need for good hold-fast sunglasses and long-billed boating caps that fit snugly enough to stay on in a gale. For wear at that long mahogany reef in the boat club the summer sailors will dig those lightweight double-breasted blue blazers topping-off immaculate white slacks.

With capital initials, the phrase mahogany reef is sometimes used as the name, or nickname, of an actual drinking establishment.

For example, the following is from The Executive Went Fishing, by Bob Wacker, published in Newsday (Long Island, New York) of Sunday 25^th May 1986:

Like an osprey’s bill, his bony nose curls over a bristly moustache. He is all gristle, skin, bones and cigaret smoke. And if you’ve got $550 to spend on a day’s sport, he’ll meet you at a Lake Montauk bar he calls The Mahogany Reef and take you to some of the finest fishing you’ll ever enjoy.
Meet Art Munday, 71, who quit being a garment industry executive 13 years ago to take other people fishing in a 40-foot cruiser, the Darius II.

Incidentally, the noun mahogany sometimes designates a bar, i.e., a counter in a pub, restaurant, etc., across which alcoholic drinks are served.

For example, the following is from Dooley Has a Symposium: Hennessy Instructs McKenna on Coinage and the Drinks Are on the House, by the U.S. journalist and humorist Finley Peter Dunne (1867-1936), published in the Buffalo Evening News (Buffalo, New York) of Monday 8^th June 1896—reprinted from the Chicago Evening Post (Chicago, Illinois) of Thursday 4^th June 1896:

“Well, I dinnaw,” said Mr. Hennessy, crossing his legs and laying his glass down slowly on the mahogany.

Skiers use a phrase similar to mahogany reef, which is mahogany ridge.

This phrase occurs in The Weekend Diary of a Snow Bunny, about a weekend at Big Bromley, Manchester, Vermont, by Elaine Cunniffe, published in the Daily News (New York City, New York) of Monday 3^rd December 1951:

Mahogany Ridge Runs.

Each lodge or inn has its own method of approaching the subject of entertainment, but we prefer to make our own. After dinner by candlelight and plaster cast, we sit around the fireplace, not exerting ourselves any more than necessary. And as the ice cubes melt, each novice talks an expert downhill run. In the game room, cellar bar and on the dance floor, tall tales turn eggbeaters into parallel christianas.

The phrase mahogany ridge also occurs in Getting the Jump on the Ski Season, by Jerry Kenney, published in the Daily News (New York City, New York) of Sunday 20^th November 1966:

Indoor Action

Despite its popularity, skiing in itself isn’t always the big attraction at a resort, because half the single gals and as many guys who make the scene do their best and most serious skiing before a roaring fire or over mahogany ridge. Some are drawn to the swimming pools and sauna baths.