My son was older than your daughter when I began diving but I did spend 4 months diving and breastfeeding. There were no more problems diving and breastfeeding than doing anything else away from baby and breastfeeding.
Having breastfed 3 children here are my words of wisdom for you...
To keep your milk supply at its optimum - stay very well hydrated and relaxed.
Don't confuse the 3mo growth spurt for a drop/lost milk supply. They all go through it, so don't blame the diving or stop breastfeeding.
Time your dives and breastfeeds/expressing thoughtfully. I don't know what type of exposure suit you wear, but diving in a 2 piece 5mm wetsuit is awful with engorged/full breasts. So if you are going to be geared up for a while, or if it is a while since you fed or expressed, certainly consider expressing before gearing up. Personally I expressed for comfort and tossed it away, but my son was older and on solids as well as breastmilk.
When my son was older I went on a weekend liveaboard for my AOW. I had to express each day/night for my comfort, and I took some antibiotics with me because I knew that if I got mastitis onboard that I would be in a fair bit of pain by the time we got back. I didn't need them but I felt better that they were there.
So ... no it did not effect my milk supply. I wouldn't expect it to affect milk supply any more than going to the gym or to work. Just remember ... drink plenty of water, stay relaxed, if you are going to miss a breastfeed - express (both for your supply and your comfort)
As far as discomfort goes - just remember to plan not to get overly full breasts while you are geared up. You may find your exposure suit a bit of an annoyance if it is tight as you have only been feeding for 3 months, but I am sure you will either get used to it or sort it out.