G'day Anil, welcome to the forum. There are many avenues open for qualified tradesmen on board ships. As you mentioned there are merchantmen; there are also the yachts of the rich and famous and the lesser ships which are found in charter fleets.
If you wish to work on the mega yachts, I suggest you look at shipboard courses dealing with safety and seamanship. Most maritime agencies (government and private enterprise) offer courses which generally have universal acceptance.
To join the merchant marine, I suggest you make enquiries before registering as crew. This goes for oil platforms also. Look at the specialist international marine employment agencies which will be well represented in your country.
In the charter world of 35-60 footers, maintenance engineers are well regarded, worked hard and get to travel, but more importantly they get well known. Good, reliable and stable MEs, are hard to come by even for big companies such as Sunsail and Moorings.
Have a good long chat to your cousin to get first hand information on the professional side of things. Otherwise, get to a few sailing clubs and look for crewing poisitions on recreational yachts. It's a great way to learn the nautical life and could well lead to a long and rewarding career afloat. Best of luck and please, keep us informed.
"if at first you don't succeed....Redefine success"!