Probably more than you want to know, but here it is:
Mechanical engineering would be more useful than electrical engineering as far as helping you tweak or fix scuba gear. I suspect mechanical aptitude is more important than engineering knowledge, though. I know an engineer or two that can design some fine equipment, but they'll break it if they touch it.
Entry level engineers are almost always salaried. Companies generally expect salaried engineers to work 50+ hour weeks, and they usually don't pay overtime. You can strike the o-word out of your vocabulary if you are a salaried engineer with "senior II", "principal" or "executive" in your title. If you invent something that is even remotely related to anything your company does, the company gets the patent. There are fringe benefits such as medical, dental, etc that the company will pick up the tab on for you, you will usually pay extra for covering spouse and kids.
Salaried engineers typically start off with 2 weeks of paid vacation per year. At my company, your vacation and 1 week sick time are in a single account (so you start out with 3 weeks per year time off. You should end up with at least 3 weeks paid vacation after 5 years. How it accrues after that varies, and typically maxs out at 5-6 weeks per year. My company limits the amount of vacation you carry forward into the next year to 1 week, which I believe is a common practices.
This, of course, assumes you stay with the same company long enough. If you switch jobs (voluntarily by quitting or forcibly by being laid off), you will have to start over with two weeks of vacation plus whatever sick time you're allowed. The harsh fact is that the easiest way for an engineering firm to save money is to pay less out in wages and salaries. If you end up developing an engineering skill on the job that for some reason is in high demand, you can sometimes negotiate for more vacation up front when you switch jobs; but you pretty much have to walk on water, and you are likely to be the first to get dumped in the next lay off. If you're that good, you should consider becoming a contractor or a consultant.
Once you get enough experience to be obtain the word "senior" in your title, you have the option of becoming a contractor (requires switching to another company). In the engineering world, a contractor is usually paid hourly and usually goes home with more money per hour in his/her pocket, though they may not get medical/dental benefits or paid vacation. Companies may go so far as to not allow contract engineers to work more than 40 hours because they have to pay them for the hours they work. Contractors are the first to get the axe when it's time for lay offs.
As a contract engineer, if you are willing to move, don't buy a lot of toys (travel light), and are disciplined enough to build a rainy day fund for being unemployed between contracts, you can work the system and do very well for yourself. Sometimes the company will compensate you for moving expenses. Sometimes you can take a temporary job out of town and get paid per diem for up to a year, at which point you quit or decide to move there. (In the mean time, you've probably already moved into a apartment there and replenished your rainy day fund.) You may be very busy when you're working, but if you build up your rainy day fund you can take several months off for vacation and casual job hunting. Networking is required, and you must build a good reputation for yourself. Collect as many written references as people are willing to give you, keep in touch with as many folks as you can, and don't burn a lot of bridges...your ability to get your next job may depend on this. You go where the work takes you, but maybe look harder where the diving is good. If you want get married and intend to live this lifestyle (really as much a lifestyle choice as a career choice), you must choose a like-minded spouse (always willing to relocate and willing to travel light). This lifestyle isn't for everyone, certainly not for packrats or folks who want to put down roots.
If you get the next level of promotion past senior ("Senior II", "Principal", "Executive", etc) and have developed expertise in an engineering discipline in high demand then you may have a future as a consultant getting paid a lot of money and have a better ability to take off all the time you want to. If you have also been published in a technical journal, have an advanced engineering degree, or have invented something in high demand, you may have a future as a consultant getting paid an obscene amount of money and have the ability to work whenever you darn well feel like it.
As an executive engineer or a consultant, you may be able to take long business trips to locations where you can dive when you're not working. Problem is, you may not end up with much play time on a business trip. If they are flying you out there because of an ugly situation/problem, you'll probably be pretty busy. If they are flying you out to improve their performance, you have a fighting chance of being able to get in some dive time. Though taking a week of vacation after the business trip ends is always an option (moving the airline departure date out to correspond with the end of your vacation generally isn't a problem).
