What is requirement in your state, and do you always keep them with you
while boating?

In Maryland you're required to have both an ID and the boat's registration
on board. I don't know the specifics of who's "allowed" to demand this
information, however. Fortunately the various officials working our local
waters seem to do a good job of not annoying us. I'm sure other
jurisdictions have their share of issues.

I think the hassles here have more to do with the patchwork of licensing and
registration than anything else. Several states do not even have decent
titling or registration of vessels.