i taking a diploma in environmental and marine science but i have interest in animals and wildlife so after i get my diploma i want to take a vet course in uni so is it possible from marine science i can get into vet?
As long as you undertake a veterinary degree after your marine science course, then yes you can be a vet. It doesn't really matter what background/career you come from as long as you have a veterinary degree.
You may still need to meet A Level requirements to get onto the veterinary course, though.