What diopter goggles would I need?

I'd suggest going and asking your optiican. I have my perscription goggles through them adn they are more able to pick a good fit and proper lens. However have you tried swimming in daily contacts? They are mively better than perscription goggles which tend to be heavy , mist up and get covered in water drops so swimming isn't easy. I've worn contacts to swim in for 40 years and never had an infection from it. I use dailies for swimming and chuck them out at the end of the day although I do swim more in teh sea than a swimming pool.