If it dissipates quickly, it’s probably steam.

    The harder you run the engine, the hotter it gets, and the hotter the water coming out of the exhaust gets.

    So it’s not uncommon to get steam, especially in colder weather.

    I would just check the engine temperature with an infra-red thermometer to make sure the engine isn’t over-heating, and maybe change your impeller and clean out the heat exchanger tube stack, because if some of the tubes are blocked, that will restrict the flow and make the engine run hot.

    Also, if you have a saildrive, make sure that the water inlet in the leg isn’t partially blocked. This can cause lots of steam. When you take the pipe off the seacock and open the valve, you should get a big flow of water (like a fountain). If you don’t, then get your dinghy hose, stick it over the hose barb and give it half a dozen pumps. You should then get the proper water flow. Continue until you do.

    Hope that helps.