This can be a little vague.' "Off the beach" would probably mean away from the beach. It could be near or far from the beach, just not right on the beach. "By the beach" would tell me that it is very near the beach - you could probably see it. If I said "JUST off the beach", it would mean the same thing as "by the beach" or "very near the beach"

The place is off the beach“ and ”The place is by the beach" are very similar , meaning that the house can be opposite to the beach or close to it.