I've seen this page before but I want to know the BEST enchantment levelt to get both on my helmet
Getting both on a helmet would be pretty rare. I looked at the most probably result to get respiration III and it looked like an iron helmet with 38 XP levels spent was 45% chance to get respiration III. Looking at the chart for that level though showed only 5 of the 25 top results having both of those enchantments, with a net chance of roughly 3%. http://pernsteiner.o..._Helmet_38.html
I would certainly prioritize getting respiration III and getting by with an efficiency V shovel or pickaxe for underwater work.