Similarly to @ToddS , I installed the Isotemp SPA 15 (15 l/~4 gal) as a replacement of the original 6 gal (rectangular one, Kuuma??). I intentionally used a smaller heater to gain a bit of space around it. I installed it a year ago in the spring, so 1.5 seasons ago. It works well, keeps the temperature and warms up quickly.
It comes with the temperature limiting valve (this model, you can get one without), which is well worth it (prevents scalding; the temperature can easily reach almost the same as the coolant temperature in the engine (180 F/85 C)).
No experience with using the electrical element, as our Club does not have shore power. But it works great with the motor coolant Hx heating.
Consider installing SS water hoses for the first 30-50 cm of the water lines (a tip from @Maine Sail ).
It comes with the temperature limiting valve (this model, you can get one without), which is well worth it (prevents scalding; the temperature can easily reach almost the same as the coolant temperature in the engine (180 F/85 C)).
No experience with using the electrical element, as our Club does not have shore power. But it works great with the motor coolant Hx heating.
Consider installing SS water hoses for the first 30-50 cm of the water lines (a tip from @Maine Sail ).