Hello Colin, I own a 2000 Hunter 310 and had the exact Water Heater issue you are having on your H290. My Water Heater tank was leaking when I bought my boat about 5 years ago and I was told by the previous owner that he was told that the thru-hole valve for the galley sink would need to be removed to get the Water Heater out. My tank is also located under the settee on the table side. The Water Heater that was installed was a Seaward 6 gal. which is built with a aluminum tank contained in a square metal housing. I was able to remove the metal cover, which made the whole unit a little smaller. I was then able remove the rest of unit without removing the thru-hole valve or cutting any of the cabinets under the settee. I did need to move the water pump and some water lines.
The new Water Heater I choose was a IsoTemp SPA20 115vac 5.3 gal. It's has a round tank shape and the capacity is a little less but it fit! It also has a stainless steel tank which should last longer and it works great. When I'm connected to shore power, it heats up quickly. When away from shore, the engine coolant heats the water and the water stays hot a long time after the engine is shut down. I also replaced the coolant lines that go to the engine. My boat is 20+ years old so when ever I remove a line I try to replace it and the hose clamps.
Hope this helps!