Stranded wire is a Federal requirement for all new boats. Has been since 1978.

Sec. 183.425—Conductors: General

(a) Each conductor must be insulated, stranded copper.

Now, as I said this is a manufacturer requirement, technically the boat owner is not required to change over if the boat already has solid wire, but it is unlikely that surveyor will overlook it. I would suggest that when you get around to rewiring that you use UL boat cable that meets UL 1426.

It is also a ABYC requirement as well.