I would say stretch your chest, shoulders and upper back muscles to make sure they are flexible. You especially don't want your chest muscles pulling your shoulders forward. Lay on your back on a swiss ball, extending your arms to the sides, stretching to touch the ground. Foam roll your back, upper back. Your deltoids will grow along with all your other muscles from lifting weights in a balanced routine. You might see some specific deltoid gains by emphasizing them in your routine, but, overall, they'll probably just grow along with your other muscles. Then there's the factor of bone structure, which gives some naturally broad shoulders and others not, and there's no way to change that. If you work your deltoids hard, be careful -- in my experience they are more prone to injury than just about any other muscles!
Swimming may be fun and have a lot of benefits, but, in terms of really gaining muscle mass, no, it's not effective in general.