San Diego overall is significantly better than Santa Cruz. Santa Cruz is better in some fields, and marine biology is one, though San Diego is excellent - it's just that Santa Cruz is arguably tops in California . San Diego is significantly better in biology, one of the reasons being that the San Diego area is a major center of biotechnological firms, and the crossover between industry and academia is a major plus.
I'd say choose UC San Diego. It is significantly superior to UC Santa Cruz in biology (excellent for San Deigo, good for Santa Cruz), while UC Santa Cruz is only marginally superior to UC San Diego in marine biology (both excellent).
OTOH, you can't beat UC Santa Cruz for environment and living conditions. I had a foster brother at San Diego State 35 years ago and, while visiting him, watched him boil a test tube of tap water down to the bottom. There was 3mm of green slime at the bottom of the tube when the last water boiled away and the green slime started carbonizing.
For medical school, choose San Diego over Santa Cruz, but Davis is better. Davis has a medical school, but I don't think San Diego does. Having a medical school on the same campus means there are many medical-related activities for pre-med majors to get involved in which will provide an advantage in applying to any medical school.
If there is a medical school at UC San Diego, then I'd rate San Diego as marginally superior overall to Davis for pre-med majors, and clearly superior for those interested in marine biology.