epoxy wont "eat" at another paint. depending on the color of paint you are using and the condition on the surface you are painting, a primer may not be necessary unless you need it so hide some light and dark patches, but usually epoxy paint is very heavily pigmented. but no matter if you do use a primer or not, if there is any adhesion problems with what is on the hull right now, whatever you decide to put over it can have the best grip in all the world, but if the existing layer below it lets loose, your paint is going to bubble or fall off.
epoxy paint, like regular epoxy has great bonding capabilities, but it has to have a solid base to bond to.... if the existing paint was put on correctly and is firmly adhered to the hull, then you should have no problems painting over its prepped surface with the two part epoxy alone...