HEADBOAT FISHING is a hybrid sport. It's the compromise for saltwater anglers too poor to charter but too proud to bounce sinkers from a rowboat, too poor to own a boat but too proud to walk the bridges. Its enjoyment depends not only on fair weather and fast fishing but on the congeniality (or irascibility) of the captain and the temper of his party.

In headboat fishing, you find a boat, climb aboard, pay your fare, select a spot and await the rumble of diesels. Your quarry may be 50-pound cod in sub-zero weather or five-ounce perch during the dog days of summer. Reservations are never accepted and when the boat fills up, it's "come back tomorrow."

The typical headboat operation is a bus trip to fishingsville. You bring your tackle, buy beverages and Babisco-type snacks at prices whispered by tiny signs tacked to the skipper's wheel house. The boat is beamy, in the 65-foot category, clean but not always comfortable and there is a head.

In the best possible case you're surrounded by jovial and courteous fishermen who keep control of their hooks, offer you pliers and shun 180-proof rum in the sunshine, the kind of good old boys you read about in Sports Afield, friends for a day, maybe longer. Your skipper's pedigree runs not from Ahab, Bligh and Queeg but from Toots Shor and other princes of protocol.

In the worst possible case, the decks are jammed with drunks, sociopaths and kneehigh children. After the fishing starts and the first bluegreen faces appear, a red-faced captain goose-steps the deck shouting: "Let your lines out. Bring your lines in. Change your sinker (hook, leader, swivel)." Fishermen trade insults, ensnarl their lines, elbow their way from portside to starboard, wherever the fish are biting.

During a recent expedition aboard Capt. Doug Scheible's "Bay King," a 45-passenger headboat, the pendulum swung from good to bad and back again.

There were 40 fishermen and three mates aboard when the boat departed Ridge, Md., for the middle Chesapeake. There, reportedly, lurked huge hardhead, jumbo spot and the full-bellied, 10-pound seatrout that have invaded the Bay for the first time in years.

Two hours out, on griddle-smooth water, maybe 10 miles south of the U.S. Navy's target ship (a bulking derelict the full fury of Naval airpower has somehow failed to damage) fishing began. It continued slow as Scheible scanned his fathometer for the telltale shadows of schooling fish. The marine band radio chatter confirmed it. The fish were in widely scattered clumps, with the clumps moving just ahead of predatory bluefish.

Scheible jockeyed his "Bay King" from clump to clump, from deep water to shallow, from drift to anchorage.Fish were hosted aboard, but too slowly to soothe the sweltering, wlbow-to-wlbow fishermen. So the skipper decided to drift and chum for bluefish.

An oily slick of pureed alewife chum streamed to starboard, whetting the appetites of nine-pound bluefish. Like hungry bounds they converged on "Bay King" - from starboard. Within minutes, the first starboard fisherman hollered "fish on." Then a second starboard fisherman. A third. FifteeN.

Bluefish ziz-zagged, diving porpoising, crocheting the lines of the star board fishermen into great disgusting balls. But on the port side . . . nothing. One by one then, the 20 portside fishermen began squeezing between the 20 starboard fishermen until "Bay King" seemed to tilt. A young mother was tugged one way by her toddlers, the other way by a bluefish

"Who's this guy?"

"You're in my spot."

"Watch out for my babies."

On that day, at that time, the boat was not the IN place to be.

Some tips to make headboating as rewarding or a t least painless as possible:

Know when what's biting and where. For example, the mackerel schools pass Ocean City during the last week in March or the first week in April. A hundred fish per couple is only an average catch then.

Call the evening before. Ascertain sea conditions, boat condition, crowd condition and fishing condition as best you can. About 75 per cent of headboat skippers will give honest answers.

Try to anticipate and avoid crowds. Weekdays are better than weekends. June and September are better than July and August. The weekend before or after a three-day weekend generally draws a smaller crowd.

Arrive an hour before departure time to assure yourself a seat. Arrive two hours early to reserve a place along the stern rail. If the skipper anchors in a current, snarls are inevitable unless you're in the stern. If the skipper chums, you can fish to either side from the stern.

Bring your own tackle. Rental rods nd reels, while serviceable, are scruffy and stubby and loaded with 75-pound test line. Skippers like 75-pound test line because they need not gaff any fish affixed to it, nor can an angler play games with his fish and snarl his neighbors.

(Do not bring your own tackle if your rod is a 12-foot surfside, a fly rod or an ultralight anything. Light tackle can be used from headboats if there are few fishermen aboard or the fish are coming in pan size, but neither situation is preditable.)

Consider bluefish the piranha of the Atlantic seaboard. Last summer, a school attacked a swimmer in Florida. On a New Jersey headboat, a fisherman tried to dislodge his hook while holding a blue between his knees. The resulting lacerations were painful and embarrassing.

Bring ice for your fish, especially bluefish, which lose flavour faster than fresh-picked corn. Nor should you count on cleaning your catch dockside, although an omnipresent boy will do it all - for about a quarter per sale.

Buy you bait at an en-route tackle shop. It's cheaper, there's a variety and the owner will advise which kind to use. At $2.50 a dozen, bloodworms are too costly for panfish. Night crawlers will work almost as well.