Remember, this is the Pacific side, not the Caribbean side of Mexico.
I was in Cabo and Puerto Vallarta at the end of March. Both were rather cold. I brought a 7mm suit and a hooded vest (we did Catalina on the same cruise), and a 4/3 wetsuit. At depth in PV I was COLD with the 4/3, as was the DM. Water temp at the surface was 70ish, but it dropped off fast. You could always take a thin suit and stick the hooded vest in your bag, just in case.
Now Cabo was cold. I wore a 7mm and hooded vest. The water temp at depth near Los Arcos was 58 deg. I don't recall what it was at the surface though. Granted, the time of year is different, but Cabo gets the Pacific current, and that stays cold pretty much all year long (at depth). And the DM had on a 7mm suit and hooded vest too, FWIW.
Pack a couple of suits just in case; it's better to be a little warm than too cold.