The 5.0 and 5.7 are the same in many ways including the plug location. It's a feel thing as they are under the exhaust manifolds but not too difficult. A mechanics mirror may help. Not knowing what boat model you have but in my 2400 SCR it's a bend over the engine job however on my past Sea Ray 220 and 200 it was a sit on the deck job.
1997 2400 SCR
5.7 Vortec / Bravo 2