The correct answer is that filters and filter housings made of 'plastic' or 'plastic' components should never be used in water that is above 170°F. ... the softening/melting point of most of these 'plastics' is ~180°F/82°C.
So, the answer is: if your engine is operating at 180°F and that 180°F engine water is feeding your hot domestic water heater and the engine is run for long periods of time ... you can assume that the hot domestic water inside that water heater can be 'close' to that 180°F ... at a maximum.
FWIW - most plumbing codes for domestic hot water recommend hot water not to exceed 120°F due to the potential of BURNS. Where the water can exceed these temps it is usually recommended that a 'mixing valve' be applied to allow cold water to mix with the effluent hot water from the 'heater' and to deliver a max. of ~120° into the distribution system. "Boat codes" usually dont have these recommendations.