Thank you all for all your good tips.
I could not see any trace of oil on the dipstick (because of the heel?). After I added about 2 quarts (more than 'some') the dipstick showed oil within the range.
There is no gauge, only a temperature alarm.
The engine is cooled with raw water. I always check the exhaust for water coming out. There was water at all times, but I understand I need to check the pump as maybe the flow wasn't enough.
I am obviously not very skilled when it comes to engines because I always thought that the raw water was doing all the cooling.
But reading your post, I am now guessing that my engine (Volvo Penta 2002) also uses fresh water. There is a reservoir which looks low as well. I will refill it with fresh water. Hopefully this was the cause of the overheating. With these hot days, maybe the water did evaporate faster than usual...
Thanks again for your suggestions.