Swimming in the open water offers unusual challenges and joys: You may encounter water snakes, jellyfish or floating debris, but the trade-offs for braving this gantlet are many: open vistas, communing with Mother Nature, an occasional dolphin sighting and a Zen-like rhythm that doesn't get interrupted by a pool wall.

I became an open-water fan last year, after doing the 4.4-mile Great Chesapeake Bay Swim (www.bayswim.com), which goes from Sandy Point State Park in Annapolis to Kent Island, Md. The difference between splashing through open water and doing laps in a swimming pool was like that between hanging by a thread off the side of a cliff vs. climbing a gym wall. It's an amazing, elemental experience that you can do solo or, for motivation, at an organized meet.

What to Expect: A less-controlled environment than the swimming pool at the Y. While lap pools are usually warmed to about 80 degrees, you're likely to find lakes, bays and oceans in the mid-'70s or lower depending on the weather. Also missing is the built-in navigation system of lap lanes; tides and currents can push you one way and suddenly thrust you back the other way, as if someone has tugged your legs. In addition, waves crashing in your face can make breathing tough and leave the seasick-prone feeling woozy. (Dramamine helps.) No matter how much lap swimming you've done, you may find yourself tiring in the open water; you don't get the frequent rests that come with turning at the wall. Tom Denes, author of "The Waterproof Coach: Waterproof Workouts for Fitness Swimmers," recommends practicing with longer sets (six laps of 200 meters long each) and never touching the wall during sets or set breaks (tread water instead). In summer, he creates a mock open-water environment for members of his Ancient Mariners Masters Swim Team (www.ancientmariners.com): Participants spend part of the practice in a circular river-raft pool at Martin Luther King Swim Center in Silver Spring, going against the current and dodging inner tubes. For longer swims (more than five miles), you should train in open water at least once a week.

What to Bring: Many swimmers go for neoprene wetsuits for warmth and to reduce drag. If you wear one, use a non-petroleum-based sports lubricant to prevent chafing. Larger eye protection (such as Aqua Sphere's Seal goggles, a cross between regular goggles and a diving mask) is also the norm.

Cost: Free if you swim by yourself; about $25 to $65 at meets.

Cari Shane Parven

UPCOMING SWIMS

Visit www.oceanswims.com for a full list of yearly events.

Captain Craig Swim. Ocean City, Md. 410-289-7556. www.openwaterswimsocmd.org. July 10 at 6:30 p.m.; body numbering begins at 5 p.m. One- and quarter-mile swims. You start on the beach, then swim out and around designated buoys and back to the shore. $25.

Sunfest Open Water Swim. Ocean City, Md. 301-645-4405. www.sunfestswimming.org. Sept. 25: Races start at 11:30 a.m.; check-in opens at 9 a.m. One-, three- and five-kilometer swims. USA Masters or USA Swimming members, $40 to $50; nonmembers, $55 to $65.

Aquatic fanatics: Ambitious swimmers at the Great Chesapeake Bay Swim had perfect weather in June 2001.