Now you are joking cause I have dive shops in my area they do not look so miserable. There is almost 200 percent profit on an equip from mask to BCDs and I know few guys who run their own shop. Its not too bad. Probably you made all the bad choices in your career. My instructor, who is 48, is diving since he was 18 and still teahing it with lot of passion after 30 years and making decent money.
I think you are overlooking the fact that most SCUBA equipment falls into the durable goods category. In other words, once people buy it they do not replace it very often. As a result your inventory turnover is not very high.
I see that you have an enineering background. I don't know wheter or not you took business courses within the your undergraduate or graduate studies. If not it might prove worthwhile before you jump into buying a dive shop.
Also, you have to consider the fact that you probably have never worked in or managed a dive shop. From the customer's perception it might appear very easy but they only see a fraction of what the dive shop owner does.
Also, from a bank's perspective you are an unknown quantity relative to opening a new SCUBA business if you have no expereience within that industry. This might make it difficult to get the startup capital for a SCUBA business since the lack of expereince adds an extra degree of risk from the banks perspective.
All of the inventory on the shelf has a carrying cost associated with it. You have fixed overhead such as the rent (or mortgage on the building if you own it). If you have actual employees you will spend plenty of time attending to government reporting requirements along with tax filings, OSHA requirements, unemployment insurance etc.
A lot of this sounds very similar to what you are trying to leave behind as an engineer.
You also might want to Google the phrase "Opportunity Cost". This is also a very important concept when trying to make a critical business decision. It will show you that you not only have to take into account your current salary, health insurance, 401-K, paid holidays, sick days, disability insurance, dental insurance etc.
No, I am not an accountant. I am software developer/project leader/systems consultant who also has an MBA. I design and develop business application software for one of the large engineering companies.
The people in this forum have given you some very solid advice. They are not trying to sabotage your dream. They are trying to make sure you consider all facets of your career switch to avoid making a descision that you may later regret.
My advice would be stick with your engineering career and pursue the dive instructor idea as a sideline.
If your current engineering job is not that satifisfying, spend time to determine exactly what it is that is making you dissatiisfied and research the steps to change that.
There are going to be bad days (or weeks) in the best of jobs. Just as panic in SCUBA diving can be dissasterous the same is true of panic career decisions.
In the end the decison is up to you AND your family.
I wish you the best of luck regardless of which decision you choose.