Zoology is a decent major if you want to work with dolphins but working in rehabilitation is a fairly new field and as such there is not a clear cut career path to get there. I can tell you it is one of the most competitive careers you could choose. Zoology may not be ideal actually; zoology is the study of animal taxonomy, evolution, physiology and behaviour, rehabilitation is less about studying the animals and more about taking care of them. If you did a Zoology degree and then did an MSc focussing more on physiology or an MSc course like Wild Animal Health: http://www.rvc.ac.uk/Postgraduate/Courses/MScWildAnimalHealth/Index.cfm
It may be better to do a more veterinary or husbandry orientated degree as rehabilitation is more about taking care of animals than studying them.
The best thing to do will probably be to contact stranding centres and have a chat with the people who work there to see what their backgrounds are. Be aware that most of these centres depend on volunteers and most of them will have very few paid employees.
As I said it's a relatively new career path and many people who currently work in this sort of position probably 'fell into them' but you cannot expect that to happen to you, times have changed and anyone who wants to 'work with dolphins' faces a lot of tough competition.
As in any dolphin related job, competition will be fierce and getting a degree is only a small, and probably the easiest step in the right direction. Marine mammals will probably not be mentioned throughout your college experience unless it's in a lecture about diving in mammals, the way to go about getting a career with marine mammals is experience, experience and more experience. And be prepared that most of this experience will be unpaid. Experience and contacts to relevant people will be essential so start volunteering and interning as soon as you can.
Mote Marine Laboratory have a great rehabilitation internship: http://www.mote.org/index.php?src=gendocs&ref=College
Internship Ops - Animal Hospital&category=Education
Most stranding centres are dependent on volunteers, check out this comprehensive list: http://www.nmfs.noaa.gov/pr/health/networks.htm
This is the volunteering site from the Texas Stranding Network, just as an example: http://www.tmmsn.org/volunteer/volunteer.htm
I would also recommend you subscribe to the MARMAM newsletter which regularly sends out volunteering opportunities involving marine mammals: