To be honest, when someone mention "upgrading from GoPro to a dSLR", the first question I would ask is: How much photography/video technical stuffs does this person know? I do agree to many people here that dSLR will yield great result, but it might not be suitable for this person!
I have encounter many people who start out crawling, and want to fly right away. It's the a very similar principle to on land photography, if you give someone who has been using Polaroid a dSLR, that person would just end up frustrated, and waste money.
One can always argue to get the best setup and with practice, the person would eventually get it, but when come to underwater, it's more than that, dive trip is not cheap, hence the opportunity of "practice" will cost you lots of money, not to mention the danger if the diver get distracted, as well as damage to reef if the person are not use to handle bulky setup like a dSLR.
To keep it short, if you're not experience with the technical aspect of shooting video (custom white balancing, shutter speed/aparture/iso etc.), consider getting a high end point and shoot and grow with it.
Last, if you spend $3000 for a housing for your 5dM2 now, you can use the same amount of money to get a 4K camera next year, bottom line, keep it simple for now, but not too simple like the goPro.