Hi Carolyn and Nick! Welcome to the best sailing forum there is!
High on insurance? What were you quoted? did you get a Survey I needed one for my boat. It actually went down last year (the insurance not the boat.)
Nick, please! don't put that "heater" in your boat! (hydraulic cement is NOT refractory cement)
You also need a secondary type of plate above the fire that will burn the unburnt gases and residue. Old timers called it a smoke shelf but not quite the same, it needs to be hot enough to work like a secondary burner. also a grate or shaker grate for the ash to drop under the fire, and a drawer... and a... and a...
Not to scare you.. but to scare you, I've seen creosote fire and oil soot fires in boilers, chimneys and flue pipes. Creosote will be a problem in your design, unless you use coal or charcoal. Which of course make deadly gases.
Pics of the boat are looked forward to by everyone here!