Kevin you have an old boat with (had) old battery cables - as wire ages - it may tarnish - which will increase resistance which will decrease voltage at the starter which will increase the load on the battery / and cable which will create heat which will increase the resistance in the battery cable and ever other component in the circuit and decrease the voltage and so on and so on until something fails
so by replacing the battery cables - you have eliminated the resistance in the cable due to age / tarnish
next 2 problem areas to surface sooner or later will be the starter and or the starter relay (and the key switch itself)
but @ least you can now take the cables out of the equation when you need to trouble shoot similar issues down the road
all too often we blame the other parts mentioned above swap them out and still have the same problem - because the cables were the root problem, good job
Dan T