You might have a look at the diesel fired units such as Espar and Webasto, originally made to keep truck engines from freezing up in European winters. They're quite compact (the heater unit itself is the size of maybe two shoeboxes, plus the tank itself), and draw little juice (maybe 8A for the 10-15 minutes it takes to make water uncomfortably hot).
If you go after the propane unit, make sure you understand issues like venting, carbon monoxide buildup, and potential for explosion in a boat. There may be a reason why the units are not approved for marine installation.
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