I'd like to add some generic info to that. While it could be a bad problem - odds are it isn't. It is a 1995 Yanmar - which in boat years is a brand new engine
and Yanmars are, generally speaking, very reliable, tolerant of abuse (which they receive as any other boat component) and usually fail badly quite rarely.
I think since you are new to engines, it is certainly worth asking the mechanic to come take a look. Personally, I'd bet if the yard did some work on the engine previously, they simply did not properly clamp a hose or something of this nature. Happens all the time. Let them fix it, and then learn more about the engine and try to maintain it yourself when you can.
As a side note - boat repair people of all kinds are not particularly prompt (I put it as nice as I can)
So, asking any of them to come out immediately is both unlikely to work and counterproductive. I am a fairly impatient person, and like things to be done quickly (especially if I pay for that) but when it comes to boats - an old adage holds "lack of planning on your side does not constitute emergency on mine", or rather "nothing on your side constitutes emergency on mine". So, take a deep breath, go sit on the stern and drink a beer, and have mechanic look at it tomorrow when he's back at work. It will be all right eventually.