Re: 13ft runabout?
No way to answer. Glass over plywood is strong and waterproof.
But that's not what makes a boat of any size "seaworthy." That's driven by 100 factors, including design, load, operator, propulsion, as to the "worthy" part, and wind, wave, swell, chop, spray, current, fetch and direction of travel relative to same for the "sea" part.
Length has very little to do with anything; look at the small kayaks running big whitewater. And "runabout" has too many meanings for different people. With small boats, sometimes what appears to be an insignificant detail can be the determining factor of "seaworthiness" for certain conditions--the shape of the rails, slant of the sides, stern deadrise, bow flare, etc.