Does marine air conditioning heat as well as cool?
Nearly all do. Reverse cycle pulls heat from the seawater, efficiently down to about 7 °C water — below that a diesel heater is the better tool.
Does marine air conditioning heat as well as cool — Explanation
Almost every marine unit sold in Australia is reverse cycle: flip the valve and it pulls heat out of the seawater rather than dumping heat into it. It is efficient — roughly 3 kW of heat for 1 kW of electricity in 12 °C water — and the heat is dry, with no combustion products and no added moisture.
Output falls away below about 7 °C seawater. For Sydney, Melbourne and Hobart winters aboard it is genuinely useful and often the better half of the argument for fitting a system at all. In Bass Strait in August, or at anchor away from shore power, a diesel heater is the tool that still works.
Equipment Specifications and Installation Checks
This answer is a practical starting point for Australian boat owners, not a vessel-specific instruction. Confirm the unit manual, electrical supply, seawater layout and current product specifications before spending money or changing the installation. Refrigerant and fixed 240 V work require appropriately licensed people.
Use the official resources for licences, standards and manufacturer documents. If the answer does not fit your boat, ask Gabriel. Advice can be incomplete, out of date or wrong; see the disclaimer.