Curious about Earnings in merchant navy

I am almost 18 now and I'm planning to go for merchant Navy mainly for money reasons. Because first, the pay is good and second, it comes with no tax. I just wanna know if there's a catch to it. Like I will get a good pay right ? It's notblike only top companies give good payment and average companies give you very low payment and you're gonna have to work your ass off your entire life then on. Please guide me