Swim laps - but, not just normal laps. Swim on your back, using legs only , with an elementary back stroke leg kick. When you can do that easily - carry a weight on your stomach. Do side stroke , legs only .
Do laps where you swim part of it under water, then surface to finish the lap. Go to a gym and do exercises to strengthen your legs.
Also, when towing - stretch out your arm - the longer you stretch things out - the higher in the water you'll be and it'll be a little easier.
If you get certified , you'll never have any problem getting a part time job.