an unpressurized heater for water or another liquid includes a shell which may be substantially filled with liquid to be heated and within which is immersed a fire box which delivers hot combustion products through fire tubes above the fire box which are themselves surrounded by the liquid contained in the shell . the fire box may be fired by gaseous , liquid or solid fuel . a secondary heat exchanger or heat exchangers within the shell also immersed in the liquid can receive salt water or other fluid which is heated proportionally to the temperature of the liquid in the shell and is circulated through a diver &# 39 ; s suit and is ultimately returned to the sea after warming the diver . back - up heating elements are included for diver applications along with required controls and instrumentation in dry compartments . built - in fuel storage tanks complete a self - contained system .