Sore Lats

Hey All, I am new here, so I apologize in advance if this is the wrong place to post this question. I started swimming a 3 years ago and have made (from what people are telling me) huge improvements. When I first started swimming I was barley able to hold 2min/100m (Ya, I am from Canada). Now, 3 years later, have brought my 1500m time down to 19:43 (1:18/100m). When I do a 1500m time trial I always end up finishing with extremely sore lats. I think if I could strengthen my lats I would be able to swim faster. I was wondering if weight workouts would help this, and are lat pull-downs best? Thanks :)