It is possible to max out an account at level 65 in Boom Beach without spending money, as long as you know what you are doing. I maxed out one account having spent about $150 USD and nearly maxed my second account for free before moving on to Narcos. They only major differences worth mentioning between the two games are that there are 8 landing crafts, compared to 4 squad camps in Narcos. Also the flare (focus) ability works a little different in that there is no cool down period but it takes battle health (gunboat energy in Boom Beach) to use it. As was previously mentioned, gold (diamonds in Boom Beach) are handed out freely on your map and are much easier to come by than gold in Narcos. Task Force operations (wars in Narcos) last 1 day, as opposed to 2, and involve taking down multiple large bases together (similar to compounds in Narcos). The end game in Boom Beach is mastering smokescreen and zookas (RPG soldiers in Narcos) together in order to head straight for the hq (finca in Narcos) and take it down while shocking all the major defences. If you enjoy Narcos, you should enjoy Boom Beach. All the defences and buildings are basically the same in Boom Beach, as well.
Take a look around the wikia and you will see many similarities. Defences all the same, resource buildings all the same, prototypes = guerilla base defenses, talents = statues, heros = sicarios (sorts but big differences here), skills are fairly original to Narcos, outposts = blackguard bases.
http://boombeach.wikia.com/wiki/Boom_Beach_Wiki
Artillery - Missile
Barrage - Fire Bomb
Flare - Focus
Medkit - Medpack
Shock Bomb - Flashbang
Smoke Screen - Smokescreen
Critters - Paratroopers
Propaganda - nothing similar in Boom Beach (original idea?)