There is a shuttle that will run you from the slips and docks to Defiance House. Executive Services rents boat slips at Bullfrog. I'm not sure if they would allow courtesy use while you are dining at Defiance House. Here's their number (435) 684-3062.
If they say they charge a fee I'd call the restaurant and talk to the manager, it seems like courtesy dock use would help increase business for them see if the restaurant manger has some influence. (435) 684-3000