I’m having Diving thoughts, I’m 42, Diabetic, smoker, fat guy (but I’m losing weight) and I have bad knees. Tell me I’m not the Diving kind and I’ll find a new thing to obsess over.
Unclviny
You might be alright for shallow clear water dives (if a diving physician clears you), but you have quite a bit going against you if you want to make it a full time hobby.
If you’re diving in cold water, a dry suit will be a must, and the added weight that comes with it. Even with a single 12L steel tank, you’ll be looking at 20-30kg of extra weight on your back. Naturally, it gets worse with doubles.
Diving also involves a lot of lifting and occasionally having to drag a lot of heavy gear some distance. Not the best thing for your knees.
Your buddy is also relying on your ability to rescue him from a bad situation, which may include dragging him out of the water. If you can’t cope with that, your buddy might as well be diving alone.
Next, the smoking would have to stop. The damage you’re doing to your lungs, as well as your (presumably) poor physical condition will give you an air consumption that is out of this world. Most technical diving manuals demand that divers not smoke, as well as recommend at least three 30 minute cardio sessions per week. High air consumption won’t kill you, but it will give you (and your buddy) shorter dives.
On the other hand, you could see diving as a bloody good reason to get back into shape 
So get yourself checked out, find out how much weight you need to lose and if you can strengthen your knees, and go for it.
You might want to look over the DAN (Divers alert network)website
Here is the link on diving with diabetes
I am a 41-year-old smoker with a bad back. Diving’s fine for me, and one of the most enjoyable things I’ve ever done.
That said, I am pretty fit considering, still have fairly good lung capacity, and do quite a bit of cardio.
I have PADI Open Water and SSI Advanced Open Water.
I have had back problems exacerbated by carrying the tank too far out of the water, but once in the water it’s plain sailing, if you’ll pardon the allusion.
Yes you need a little bit of fitness - you need to be able to swim a certain distance and lug bits of equipment, and possibly your buddy - but you don’t have to be super-fit. And you’ll also need an all-clear from a doctor.
If you’re concerned about strength, just don’t dive in situations where you’ll need it - dive with an instructor in a group accompanied by divemasters, for example.
I do exersize regularly, I walk for an hour every other day and on the other days I do 100 crunches, 50 pull-ups and 5 leg-lifts.
I think I’m more interested in fresh water diving than offshore and either way we are talking a few dives a year maybe. My “hindered mobility” on Land is one of the reasons I’m attracted to Diving.
Stopping smoking is not going to happen, I’ve had to give up too much already.
Unclviny
The biggest issue IMO is diabetes.
You can dive where you fall into warm, carribean waters, float around without any real workout for awhile, then get back out.
If you mess up with the diabetes where you get an attack of the stupids or actually pass out, that can kill you FAST.
At the very least IMO, you need a partner that will MAKE sure your blood sugar is good, you are doing the right things before the dive to make sure it stays that way, and watches you like a hawk during the dive as well.
Underwater is NOT a place to have a brain fart or pass out.
You’d also need to be extra careful regarding observing no decompression limits, doing things to lminimize the chances of an “undeserved” deco problem, and generally just not getting hurt, as diabetes is going to compound any of those problems should they arise.
Do some serious research and best of luck!
You should at least try it out - if you don’t want to take the full Open Water course there are Discovery Scuba sessions to give you a taste of it. If you end up liking it, you may have a few risk factors to keep in mind but nothing that HAS to keep you out of the water.
You don’t have to ever do the strenuous, hard-core technical diving. I got certified and addicted big-time a year ago and I’m approaching 100 dives, including getting my advanced open water certification…and I’ve never done anything other than beautiful clear-water ocean drift dives in Mexico and gentle, relaxing shallow dives in the local lake.
Risk factors - they really do depend on the individual. I have dived with many a smoker (including divemasters) and you wouldn’t know it by their air consumption. If you do cardiovascular activities that will build up your lung capacity, meaning you will use up less air and can stay down longer. Same with weight - last month I dove with a guy who was at least 300 pounds. He used less weight than I did (wow!) and could stay down almost as long as I could (a 150 pound female) - and I have pretty darn good air consumption. Sure, the stereotype of overweight divers or smokers is that they blow through their air supply in 30 minutes, and probably that is more statistically likely - but that does not necessarily mean that will be you.
Bad knees - if you go on shore dives, you will have to be prepared to lug equipment, which can really wear you down depending on how far you have to lug it. But if you’re diving at the local lake where you can pull your car close to shore, you won’t be moving it too far. When I go on vacation in Mexico, the dive shop lugs my gear out to the boat, sets it up for me, and all I have to do is backroll off the boat, drift with the current, and climb back out at the end. Major easy!
The only thing I don’t know much about is the diabetes. You might check www.scubaboard.com - they have an active membership that can give advice about many things. As other posters suggest, it is best that you get checked out by a physician before undertaking scuba anyway.
Final note - scuba is a risky sport by nature and you do have to be aware of the things that can happen and how to best prevent them - but you shouldn’t automatically exclude yourself based on physical factors until you’ve given it a try.
I should add, by the way, I’m a bit biased =) I dive in the Baltic and North Atlantic, so freezing water with 2-3m visibility is my norm.
Novalyne’s advice it probably what you should go by.
I concur with Novalyne. Be aware of your personal risk factors, and manage them according to your risk tolerance.
And definitely start with a discovery / resort course before you invest time and money in certification and gear. (Scuba is a money-intensive activity.)