This is a chamber to collect any water that remains in the exhaust pipe when the engine is stopped. It also acts as a baffle and silencer. Made of GRP, plastic or stainless steel, it must be positioned below the engine exhaust outlet, at the lowest point of the exhaust system, so that water can drain to it.
A plastic water trap can melt if the engine is run without cooling water. This can result in exhaust fumes collecting in the boat, heat build up and possibly fire. An exhaust temperature alarm sensor can be fitted to the trap to indicate loss of water flow. GRP and stainless steel water traps will withstand higher temperatures.