There is often a resemblance in nature of one species to mimic the other,the ringed water snake being one of those,recent flooding brought a lot out of shoreline grass,brush.Many of them relatively harmless but not all,last time out I had one that wanted to rest I guess tried to climb in the splashwell,minor rod push and it gave it up.
And yes a juvenile cottonmouth would resemble a copperhead,the main difference between it and a non venomous snake is head shape,ace of spades.
When out of the water an adult resembles more a timber rattler as is dark with brown verigated marking.sometimes greyish but one cannot mistake that open mouth posture if disturbed.