Few things are more pleasant and exhilarating to watch than a nice Chesapeake Bay line squall, as long as you can watch it from shore. I had that delight recently after a day of hard fishing on the lower Bay. As my partners and I filleted 28 (count 'em) monster bluefish of 10 to 15 pounds, we watched the sky turn black and tempestuous, and the light chop at the mouth of the Potomac turn into a wind-whipped froth. We were celebrating our good fortune and timing when a marina worker rushed down to the dock and scanned the water for a boat that had radioed in a distress call. Soon enough she turned up, a 25- foot sportfisherman, bow-heavy and taking water as she lurched along in the foam. There was a rush to bring her dockside and get the pumps aboard. The man at the helm, alone, had a wild look in his eye that told you these storms could make a believer of a heathen. I can vouch for that. Three Junes ago on the Potomac, just below Wilson Bridge, a line squall roared out from the Crystal City skyline and dumped my colleagues and me out of our Lightning. The river bottom ground the mast of the little sailboat into tinfoil while we clutched the upturned bottom, listening to the grinding and the wind howling and bomb blasts of lightning shattering trees in the woods on shore. You get so scared you're beyond scared, if that makes sense. The mind acknowledges, "I am in the hands of something so far out of my control that if this is my time, let it be." Someone died in that storm not far from where we were and a fellow drowned after another unexpected line squall struck early this spring and dumped scores of unsuspecting Sunday sailors into the still-frigid Potomac. That more-recent storm took almost everyone by surprise, coming as it did a month before the normal start of squall season. But from now until September, high squall season, no afternoon thunderbumper should come as a surprise to boaters. Not that there's much you can do about a line squall even if you know it's coming. The best thing is to run for cover, which works for a fast powerboat with a little warning. The next best thing is to prepare to ride it out, which on sailboats means shortening sail and being prepared to drop to bare poles in a hurry. On powerboats, it simply means battening down and finding something solid to hold on to. In all cases it means donning life jackets. To be prepared for a summer storm, it helps to understand what causes one. Usually it's a product of heat. A burning sun beating down on the Bay or river all day heats the air, which starts to rise. By midafternoon, so much heated air has risen that it creates a deathly stillness at sea level, essentially a vacuum left by the departing warm air. Nature, you will remember from junior-high science, abhors a vacuum. Air comes rushing in from wherever there is some. Wind is an automatic result and, if the air is considerably cooler or wetter or drier or in some other way different from what it's replacing, it creates rampaging turbulence, electricity and precipitation. To see one of these storms advancing across the water is to be humbled. A good line squall is awesomely powerful. The raging air whips the water into a vertical wall pushed along by evil-looking clouds and erupting with daggers of jagged lightning. Sometimes waterspouts dip down from the overhanging clouds and simply slurp up everything on the surface. Even Caspar Gotrocks in his 65-foot motor yacht with the Onan generators and the twin V-12 diesels says his seaman's prayer then: "Oh Lord, your sea is so big and my boat so sm information for "Baltimore Harbor to Patuxent River and the tidal Potomac," or "Cape May to Cape Charles and 20 miles offshore." Usually these people know when bad stuff is coming, and that can provide a vital head start. The National Weather Service also has some free brochures that explain about summer thunderstorms and tornados. Address your request to Richard Schwerdt, National Weather Service forecast office, World Weather Building, Room 302, Washington 20233..