Your question is a two part one. You need to determine whether you want to go deck or engine. Sea time is counted toward one or the other. NOT BOTH. Unfortunately time on the yachts that you have will limit you to a small license. Maybe 100 ton or so. Or maybe a engineers license for 1000 HP. There aren’t many good paying jobs right now for the small licenses. Look around this website, there are many threads from sailors with years experience lamenting the lack of jobs.
Working in the engine department can be both an Unlicensed position, and a licensed position. It depends on what type vessel you are on. Usually the definition that requires you to be licensed or not is the particular vessels ‘Certificate of Inspection’ (COI.) Not every boat IS inspected. Most are Documented (that is federally registered) instead of having state FL 1234PR (for example) numbers. Typically boats (of many sizes) are Inspected so that they can carry passengers ‘for hire’; either short trips (like ferrys) or longer term charter (like dinner boats.) On the COI it is stated how many, and what license they must have to legally crew (man) a boat. If a boat is UN inspected then by law it CAN’T carry more than 6 passengers (plus one crew) total of seven aboard. Thus the term ‘six pack’ license. Yachts typically have unlicensed engineers, since a license costs more to hire. And the captain is usually licensed, since most larger Yachts carry some passengers for compensation. It really strange how the compensation is figured out. And insurance is a lot less if there is a licensed Captain running the vessel. COI’s state the minimum crew needed to run. From stating the vessel may operate with ONE licensed operator, or two (depending on how many hours per day it will operate, to specifying how many crew, in each department are required. It just depends on how big, how many passengers, and where it will be running.
Likely your seatime from the navy would count towards an engine license, but you need all your service records, and dd214. In addition you need letters attesting to service signed by the officers who actually SAW you do the work. Do you have their contact info?
If you want to progress past small licenses (50, 100 ton; or limited engineering licenses) you will need time aboard larger vessels, with higher tonnage, and/or horsepower.
Just curious, what are you looking for a yacht career, or in the shipping, tug end? Either way, if you have a merchant mariners credential you ARE in the merchant marine! But there are many differing sectors to work in.