I can''t see how a 15'' beam is too narrow. I would think a 37'' Cat with a 15'' beam is plenty large enough. I have a 33'' Morgan OI with a 12'' beam and its large enough and wide enough to make the journey.
Maybe you can get ahold of some other Prout owners and get their direct experiences.
While Prouts have done a fair share of offshore work that sounds quite narrow for a catamarran. The rule of thumb used to determine the minimum acceptable beam on a offshore Cat was that the beam is more than half of the length with 60% being more optimum. In capsize testing it was found that a near breaking wave height equal to the beam of a cat would capsize it with a high certainty of chance. The B/L=.5 is a pretty old ratios and I don''t know where the current crop of beam to length ratios have ended up but I suspect that they are wider still. .