Thanks again, Bonnybrook!
Yes, Harima is the name of what was a large province covering those places; officially it's now just the name of a small town to the west of Akashi. But it's on the coast "just down the road" from Tarumi and I'm sure you're right. But I've tried including the town's name in a deeper research but can't come up with anything, unfortunately

曳船 (tug)　日好丸 is listed among commercial members of the Harima Town Societies of Commerce & Industry in www.harima-sci.or.jp › cgibin › contents so I'm guessing this tug has been preserved precisely to take part in events such as the Tarumi festival. The ancient Harima Province area includes such towns as Kakogawa and Akashi and I guess Tarumi would be near the eastern fringe of it.

I would say that this tug is around 20m length and probably well under 100gt. Most unlikely to have an IMO number

Thanks, Donnybrook.
But that one was built in 1992 & the one in my photo looks older than that. The name could also be pronounced Hiyoshi Maru. With this name there is one (IMO 5151701) built in 1953 which looks old enough but is classed "decommissioned or lost", and another (IMO 751006) built in 1975 which is a better bet (balticshipping & others has her flag as China but they've been wrong before with ships ending in "Maru"). So without some photo evidence it's not possible to come up with a definite identification.

There's a reference to a Kobe-built tug Nikko Maru at
http://maritime-connector.com/ship/nikko-maru-9042477/