The Imai surfboats are very close to the look of a MSB. Sadly, they are the wrong scale but using Imai's boats as a reference one could fashion a surfboat out of, say, basswood or even thick plastic. As I mentioned on another site, Iron Shipwright's 1:350 scale Hamilton could be used in reference regarding the MSB. But another builder that has the kit stated the small boats in the kit looked like navy whaleboats. If you're a real Coast Guard model junkie, be prepared to scratchbuild
Mike