Sounds like an RV heater. I have one, they install from the outside. If you have flex water lines you may be be able to pull the heater and then remove the lines. Otherwise you'll have to find a way to access the inside to remove the lines.
Once you remove the cover on the heater, you should find a flange around the heater with screws/rivets holding it to the boat's structure. Remove them and the heater should slide right out.
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Willie
She's a tired old barge but she's paid for!
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