Too skinny?

Ok, so nearly everyone at work thinks I'm too skinny.

I recently started this new low-fat diet and I'm getting leaner, but not really building any mass. I bought some protein shake mix to help my body build muscle, but I don't think swimming is the best way to bulk-up. Soooo, my plan is to get a personal trainer and hopefully consult a dietitian so I can start to gain some GOOD weight.