YOU DON'T NEED A DRILL SERGEANT to call "Ten-shun!" to JAG star David James Elliott. As a recent ratings grabber showed, a well-wrapped towel around his waist is quite enough. In or out of the series-issued lieutenant's uniform, the square-jawed, aquamarine-eyed star of NBC's Navy drama has women more than willing to discuss terms of surrender. Unfortunately, such perfection can also make a bad first impression. "When I met him I thought, 'He's so great-looking, he's probably a jerk,' " says Elliott's wife of four years, actress Nanci Chambers. "But ha wasn't at all, and that was very attractive to me, his modesty." Yet even his mother, Pat, can't get beyond her 6'4" son's bone structure. "After she sees JAG, she calls to tell me, 'Ya looked good, says the Toronto-born actor, a graduate of Ryerson Polytechnic Institute, Canada's answer to Juilliard. "I'll be like, 'Ma, what about my acting?' " All the same, Elliott, 35, aims to please his viewers. "If people think I'm a good-looking guy and it makes them watch the show, great," he says. "I never say give up, especially in this town, where there's a line of guys down the street just waiting to take your place. So I find the time to work out. I have dumbbells in my trailer on the set. And when I get home at 3 a.m., I'll crank out my situps." Elliott's fans appreciate the effort. "I got one memorable letter from a 90-year-old woman," he recalls. "She said that if she were younger, she would just eat me up with a spoon!"