I have a 50 year old wooden boat (iroko & oak) and every August I find in the bathroom several, maybe 4 or 5, small flies about 5 mm long. They are not very active and easy to catch. The boat is in Turkish waters so nice and warm.
With a wooden boat one is always concerned about wood boring insects.
deffo drain fly, (owl midge) they thrive on decaying matter like the stuff in your outside drain from the kitchen sink.. go look at these areas. boiling water with a mild disenfectant will normally break their life cycle.