Looking at spending a night on the coast to break up travel from koyasan and Hiroshima. Recommendations please?
You could stay in Kobe, a port city on the Inland Sea. But a better stop is Okayaka/Kurashiki, where the latter is a nice traditional area not too far from the water.
Japan is not a beach resort country.
apart from Kobe, consider also Onomichi and Naoshima in Seto Inland Sea. Search on Japan guide for more info.
I vote for Kurashiki, a favorite of mine: https://www.japan-guide.com/e/e5751.html And do stop off to see Himeji castle, either on your way to or from Hiroshima: https://www.japan-guide.com/e/e3501.html
I'm assuming you are also going to Miyajima as part of your Hiroshima sojourn, and the iconic gate (torii) arises out of the water. So plenty of coast in Hiroshima/Miyajima.
And if you do make the Okayama/Kurashiki stop, Okayama is home to one of the three great large gardens of Japan, Korakuen.
Yes we will be going to Mijayama as part of Hiroshima
You might consider the Shiraishi International Villa, which is on Shiraishi Island, a ferry ride from Kasaoka: