Bio oil is wonderous for preventing stretch marks. In my history I went from a 250lb fatass to a lean 160lbs... During the year I dropped all that weight I got a ton of stretch marks on my stomach.
Bio oil fades them completely, though interestingly enough, the effects are temporary. Two weeks of bio oil and they go away. Two weeks sans bio oil they come back. I've tried just about everything else, short of laser therapy or cosmetic surgery, and bio oil's about the only thing that works. Should help with gain induced stretch marks too.
And of course, a diet high in water and healthy fats will help a lot as well, in just about all areas related to fitness. Do drink up and take your fish oil.