Chalk is used as a cheap filler and is rather heavy. Why degrade the properties of the epoxy when you can apply excellent quality marine paint which will protect the epoxy from UV and keep your boat shiny (and of consistent color) for a long time.
Chalk means:
1 weight
2 lot of work
3 degradation of epoxy properties
4 very average results

Adding pigment is really practical only for gel coats on large female molded surfaces or small composite parts. Paint is the best option, at least that is what I would do.

Vaclav