Against a background of salt spray and glistening rocks, Toni Frissell photographed thus seagoing swimming suit of mesh sea satin. Made with "Lastex" — sleek as a tarpon's side. Helen Wills designed it. That fabric, with its striking new lattice effect, is capable of producing pleasant optical illusions about your figure as you go down to the sea. B.V.D. suit from Lord and Taylor; Marshall Field; Neiman-Marcus