When my "new to me" E35 Was on the hard for a bottom job...I asked if they would flush the cooling system and add 50/50 marine antifreeze on my M25...it was time. I reminded them to bleed the air out of the system or the engine would overheat. The mechanic told me they did. When the boat splashed yesterday, I cranked...started right up and began to heat to 200 (never before). I shut it off, called their mechanic who did the flush, and he spent 2 hours trying to figure it out trying ways to bleed the bubbles out of the cooling system. Nada. Unhappy (being polite here) as it was getting dark and very frustrated, I started looking around. I opened the starboard cockpit lazarette and noted a radiator cap on a 12" or so square aluminum box with 2 hoses leading to the engine cooling system...I asked what is this for? He looked and did not know. I opened the cap and it was bone dry. I put 1/2 gal of 50/50 antifreeze...cranked the engine and it topped out at 180 degrees with one hose turning warm and the other cooler. Left for 30 minute dark engine trip to my marina (not an ideal time); running lights worked! Engine locked on 180 for the trip, never missed a beat. Now I need to change oil again due to the 200 degree overheat (changed the oil 3 weeks ago). Does anyone know what that radiator capped box is? A hot water heater of some sort? How does it heat the clean hot water with antifreeze in it?...had to leave due to night time and will go back mañana to take another look. Sorry for being so ignorant.