I made the pilgrimage to the boat last week. The ISOTEMP SPA 15 looks like the best fit! The cabinet opening is 335mm (13.25 inches) wide; 385mm (15") high. The real determining factor though is the depth from back of cabinet face to where hull starts to rise above the hot water heater mounting platform: 540mm (21.5") . There is about 680mm (27") to storage cubby under the sink along the port hull under the galley. But there is about a 1 to 1.5 inch rise which makes it seem doubtful the SPA 20 could fit in that space due to length of the water heater.
Water heater on order! Will post photos of installation. (There are some nice you tube video of installation on other vessels)
Daryl Clark