If you goal is to increase your dimensional width across your shoulders when viewed from the front or rear, you will need to increase the size of your medial deltoid muscle.
The shoulder is controlled by three different muscles, the anterior, medial and posterior deltoid.
The anterior deltoid (front) pulls the arm up directly in front of you as if you were marching. Boxers have very pronounced anterior deltoids.
The medial deltoid raises the arm out to the side, as if you were doing star jumps. Gymnasts tend to have well developed medial deltoids.
The posterior deltoid pulls the arm rear wards when it is in an elevated position, as if you were rowing a boat, hence rowers have strong posterior deltoids.
The easiest way to develop larger medial deltoids at home is to perform lateral raises. Stand with your arms by your sides and slowly extend them outwards until they are parallel to the floor as if you standing in a T-shape, then after a momentary pause, lower them down again and repeat.
You will need to hold a weight in each hand to give enough resistance, but it won't need to be much - a 2 to 4 pint container of milt should be enough.
The shoulders respond well to higher than average repetitions for maximum hypertrophy, so aim to to do between 12 and 15 repetitions.
if you can easily do 15 reps, you need to add more weight. If you can't do 8 repetitions, lower the weight until you can do at least 12.
The repetition should be slow and controlled, without swinging the weight up or down and you must keep your back straight.
Make sure your thumbs are pointing forwards or slightly downwards to further activate the medial deltoid - as if you were pouring a jug of water.
To stimulate growth, you need to do reps until you can't do any more in a slow and controlled fashion and you 'fail' at maximum effort. It will burn and hurt, but this it eh bit that stimulates growth and the failure point should be between 12 and 15 reps.
Aim to do 4 sets of 12-15 reps with each set reaching failure, with a rest period between sets of between 60-90 seconds.
With larger muscle groups you would typically allow 1-2 days rest for muscle recovery, but the shoulder and other smaller muscle groups are fine to train every day, once per day.
Ensure you eat a reasonable amount of protein in your diet to maximise growth, but if shoulders are just your goal, it won't need to be much.
6 weeks of this will add a few inches to your width.
Swimming is a great overall exercise for shoulder strength and mobility, but doesn't specifically target the medial deltoid and you'll have to do an awful lot of it to see noticeable results in width.