Desalination for Boats and Small Islands, Priced Honestly: A Break-Even Bound Against Trucked and Shipped Water

Small reverse-osmosis (RO) desalination units are marketed to boat owners and island communities as freedom from water logistics. This paper prices that claim in full, in dollars per cubic metre, against shipped or trucked water, rainwater catchment, and dockside water. Boat watermakers cost $15.31/m3 (1.5 m3/day, no energy recovery) and $11.14/m3 (3 m3/day, with recovery) at 30% utilisation, against $3.00/m3 dockside, and lose on price in every case computed. Island units cost $3.19/m3 at 15 m3/day, $2.46/m3 at 50 m3/day, and $2.08/m3 at 100 m3/day at 70% utilisation, beating a 100 m3 tanker barge ($5.60/m3 at 5 km, $11.00/m3 at 50 km) at all distances, provided utilisation stays near baseline; at 20% utilisation a 15 m3/day plant rises to $7.29/m3 and loses inside 19.1 km. Fully costed household rainwater is $8.44/m3 and capped at 53.76 m3/yr per 80 m2 roof at 1,200 mm/yr. The thermodynamic minimum at 50% recovery is 1.143 kWh/m3; real small units use 3.0-10.0 kWh/m3. SIMULATED, NOT MEASURED: all costs are model outputs from stated assumptions.