March is here. It’s a month when everything is green, including the beer. Dunno if the Gardner McKay play Sea Marks, which opens Friday at the Thistle Dew Dessert Theatre, is green, too (or even orange), but it most certainly has an Irish-related theme: Fisherman sees girl once, gets big crush, then entrusts the post office with the fate of his nascent romance. Nisa Davis-Hayden and Don Haden star; Adrienne Sher directs. At the Thistle Dew Dessert Theatre, 1901 P St. 8 p.m. Friday, Saturday; 6 p.m. matinee Sunday, through Mar. 31. Reservations required; call 444-8209. Tickets $10-$17, with group, senior and student discounts.