I've long been fascinated by the genus Arbutus.

Somewhere on the web years ago, I read a bona fide expert on California ornamental plantings observe that *even there*, Arbutus 'Marina' can be fussy. I'm not even sure this is a Marina, but if it is, it could be having the sort of dieback - probably a root rot like Phytophthora - that can affect them. On the east coast, with our humid, rainy summers, 'Marina' is totally ungrowable, even in places where the winters would be mild enough for it to survive. It is 50% A. canariensis, that comes from a climate with relatively cool, dry summers like coastal California. OTOH, Arbutus unedo can grow in the southeastern US, as long as they are on dry, preferably sandy soil. As can the hybrid with a Greek species, A. x andrachnoides, because the Med gets awfully warm in summer and thus dewpoints, and even the remote possibility of summer rainfall, are way higher than they are in the Canary Islands, or coastal California, with their chilly oceanic waters.

Will be interesting to see what experts on gardening in maritime climates like Gardengal have to say, but my unfortunate gut instinct is at this point, the dieback will probably continue and the plant is doomed. Would help to know what's going on on the other side of that fence and how deep that footer is. If your neighbors suddenly started growing something very water intensive on the other side and started heavily water in summer, that could have triggered this. But more likely your own lawn watering might have been the issue.

This happens in England, too, FWIW. On the old GOTE forum, I reported on a gorgeous Arbutus I saw in Rye in 2010, only to find out it was dead.

I will say too that, here on the east coast, sometimes we get just a single branch of a rhodie having some kind of rot related dieback, and can just cut that branch off. The spores are everywhere, it doesn't 'stop the spread' per se, but it's mostly for cosmetic purposes. The plant can continue on as long as the other branches don't succumb. In your case since all of the dieback seems to be on the branch coming off at the base, if it were my garden, I'd cut that off, dispose of (not compost) the branches, and hope for the best.

Also would help to know about your local microclimate. I'm going to hazard a guess that w/a plant like this, there can be a difference in outcomes between a place like Santa Barbara vs. the much hotter summers of Pasadena.

We are 1 mi from the ocean in Redondo Beach. 60s/70s summer, 50s/60s winter.

I think they especially don't last forever in typically cultivated gardens.