I am an overweight (about size 22), over 50 female diver. i love it, I am not heavy once in the water, I hope to still be doing this when I am 80. I have been on 2 weeklong liveaboards, and lots of dayboats, and no one, has ever said anything to make me feel bad, but I have gotten comments about how good I am once in the water that made me feel great.
Some of the things I have done to make it easier on myself: Have my own wetsuits (both 7 ml for California, and 3 ml, for the tropics, and a microprene skin for very warm water, or under the 7 ml when it is very cold). I bought Henderson hyperstreach for all 3 because it is so streachy that even though I am size 22 in clothes, I am size 12 (can you belive it - not since I was 14 years old!) in the Henderson, and it fits perfectly over my big stomache & wide hips. I also have my own weight intergrated BC. That way I don't have to be embaressed by trying on gear at the shop, that doesn't fit, and I think it is safer too, since I know my own gear. I also dive NITROX, and stay very well within the air (not NITROX) tables, to give myself a big margin of safety, since I too worry about DCS. The weight intergrated BC means no one knows how much weight I have on, just goes in the removeable pockets. From reading about dive accidents, the biggest risk seems to be heart attack from long stressful surface swims, so I only do boat dives, and work hard on my navigation skills, so I always come up at the lader, no surface swim. Sometimes I have missed the boat, then I use my safety sausage and make them come get me, if I am far away. If I am close I just take it slow & easy back to to boat, or go down 5 feet and do it underwater (I leave enuf air that I can do that).
Sometimes my very skinny husband gets too cold, or tired to want to continue diving so then I have to find another buddy, and this has never been a problem, no one has been reluctant to dive with me, cause I am as good as they are once in the water.
Because of my excess weight, and age, I have 3 bad disks in my back, and can't lift or carry anything over about 10 lbs. So I can't wear my gear out of the water. On every dive boat I have been on (lots) the divemasters have been friendly & helpful about bringin me my gear as I sit on the edge of the dive platform, and holding the weight off my shoulders (weight intregrated allows this) while I buckle up, then I do a forward roll into the water. When I come up, I hand up the weight pouches, then the BC, then my fins, then climb the ladder. Everyone has always been helpful and accomodating, and made this feel normal, and no one has ever said anything to make me feel bad. On my last liveaboard (at the great barrier reef) there was a woman who was much larger than me, and she had over 1000 dives, and had ther respect of everyone on board for her diving knowledge and skill.
Go for it, have a good time, and be safe. No one is likely to make you feel bad, and all will want to help you have a good time.