Wetsuit thickness definately varies person to person. I have not been there in March but was there this past June and temps varied from 66-74 F that week. My wife and I used a 5mm and hood and were a little chilly on the lower temp days.
That same trip it got as low as 60F in Cabo San Lucas and we were freezing in 5mm. Dive Master said she was surprised we stayed down so long as cold as it was to which I said, "But you are also wearing a 5mm?" She then said," Yes, but I am also wearing a battery operated heated vest as well". That's another option. Maybe bring a hooded vest and rent a suit, the dive shops in Cabo Pulmo had 5 and 7mm rentals so you can always find the perfect one for whatever the temp.
The main thing is you loved it and want to go back. I'm going back in December, beats working.