Tell her that is not only catamarans but also Dinghies, Kayaks, Windsurfers, Kite Boarders, and Stand Up Paddle boaders need a sandy beach area to launch from too. The more people that can benefit from it the greater chance of sucess.

Play the liability card. Reminder her that a small launching/landing area of sandy beach is much cheaper than paying a lawsuit to a plaintif if one slips and injures him/herself on a slippery concrete ramp.

Regards,
Bob