i've used dermablend for years to cover bruises & tattoos. i successfully covered my tattoos completely when i got married. it is water proof & can handle sweating & swimming. NOW i never tried it for stretch marks which tend to be grooves in the skin. the only draw back for me was it can LOOK different under different lighting. sometimes it looks natural other times you can see what looks like a dry spot lighter spot. try it & if it doesn't work for stretch marks you have it for bruises.
